Forgetful distribution

Algebra Level 4

When first introduced to multiplication in algebra, a common mistake made by students is to claim that a × ( b + c ) = a × b + c , a \times ( b + c) = a\times b + c, because they forget to distribute a a properly. For how many ordered triples of integers ( a , b , c ) (a, b, c) , each of which are between 0 and 10 inclusive, do we actually have a × ( b + c ) = a × b + c ? a \times ( b + c) = a\times b + c?


The answer is 231.

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26 solutions

William Cui
Nov 25, 2013

Let's calculate what a × ( b + c ) a\times (b+c) actually is. By the distributive property,

a × ( b + c ) = a × b + a × c \qquad \qquad \qquad \qquad \qquad a\times (b+c) = a\times b + a\times c

Substituting a × b + a × c a \times b + a \times c for a × ( b + c ) a \times (b+c) , we have the equation

a × b + a × c = a × b + c \qquad \qquad \qquad \qquad \qquad a \times b + a\times c = a\times b + c

We can subtract a × b a \times b from both sides of the equation to obtain a × c = c a \times c = c

Dividing c c from both sides, we have a = 1 a = 1 . However, c c can also be equal to 0 (we can't divide by 0), since it ranges from 0 to 10 inclusive.

If a = 1 a=1 , there are 11 × 11 11\times 11 possibilities for b b and c c . Likewise, if c = 0 c=0 , there are 11 × 11 11\times 11 possibilities for a a and b b .

However, there are 11 11 possibilities we must subtract, since there are 11 11 possibilities for b b when a = 1 a=1 and c = 0 c=0

Hence, we have 121 + 121 11 = 231 121+121-11 = \boxed{231}\ \blacksquare

good procedure

Ephraim Akli - 7 years, 6 months ago

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Thanks.

William Cui - 7 years, 6 months ago

I prescribe you approach to everyone

Ephraim Akli - 7 years, 6 months ago

Best approach

Amarjiit Pande - 7 years, 6 months ago

Gud 1.. I did it in the same way :)

Engr Fahim Uddin - 7 years, 6 months ago

Wow! I was so close! I forgot to subtract 11(my answer was 242) Thanks for the answer!

Kou$htav Chakrabarty - 7 years, 6 months ago

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Yeah, I actually put 241 at first because I subtracted 1 instead of 11.

William Cui - 7 years, 6 months ago

You are not alone.

I got 232 instead because of my typo.

Evan Huynh - 5 years, 4 months ago

You write "since it ranges from 0 to 10 inclusive". However the problem states "between 0 and 10." Clearly, the problem agrees with your interpretation but that's what tripped me up.

Alex Shapiro - 7 years, 6 months ago

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Yeah, but in the problem, it states that each variable is from 0-10 inclusive.

William Cui - 7 years, 6 months ago

same way !!!

Sarith Imaduwage - 5 years, 2 months ago

is ur answer correct?

Kipa Tachak - 7 years, 6 months ago

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Yes.

William Cui - 7 years, 6 months ago
Andrew Sellek
Nov 25, 2013

By expanding the left-hand side, we get a b + a c = a b + c ab+ac=ab+c Subtracting ab from each side gives us a c = c ac=c Subtracting c from each side gives us a c c = 0 ac-c=0 Taking out a factor of c gives us c ( a 1 ) = 0 c(a-1)=0 Now we know that either c = 0 c=0 or a 1 = 0 a-1=0 For each of these there are 11 options (each of the integers between 0 and 10 inclusive) for the other variable giving a total of 22. However, we must be careful to remove 1 instance of a = 1 , c = 0 a=1, c=0 as this is duplicated. This gives us 21 different combinations for a and c. For each of the combinations, there are again 11 different possible values for b. Therefore the final answer is 21 × 11 = 231 21 \times 11= \boxed{231}

Thanks. I felt it was clearest to do it this way which requires a similar thought process to solving a quadratic equation or any other factorised equation.

Andrew Sellek - 7 years, 6 months ago

I did it similar to William but I think your solution actually does explain why we took those cases in the first place.

Anoorag Nayak - 5 years, 6 months ago

i this is a better method

Kipa Tachak - 7 years, 6 months ago
Mohit Sharma
Nov 24, 2013

Since, a × ( b + c ) = a × b + c a \times (b + c) = a \times b + c is required Therefore, a × c = c a \times c = c => ( a 1 ) × c = 0 (a - 1) \times c = 0 Hence, a = 1 or c = 0

  • Case 1 : a = 1

Here, b can take 11 values and c can take 11 values => 1 × 11 × 11 1 \times 11 \times 11 = 121 triplets.

  • Case 2 : c = 0

In this case, a cannot be taken to be 1 because we have already considered that in previous case.

Hence , a can take 10 values and b can take 11 values => 10 × 11 × 1 10 \times 11 \times 1 = 110 triplets.

Therefore, total triplets = 231

Sayan Ghosh
Nov 28, 2013

include <stdio.h>

#include <string.h>

main()
{
   int a,b,c,d,e, count;
   count=0;
   for(a=0;a<11; a++)
   {
       for(b=0;b<11; b++)
       {
           for(c=0;c<11; c++)
           {
               d=a*b+a*c;
               e=a*b+c;
               if(d==e)
                     count++;
            }
        }
    }
   printf("%d", count);
}

So the solution is 231 \boxed{231}

Daniel Chiu
Nov 24, 2013

Expanding, we get a b + a c = a b + c ab+ac=ab+c so a c = c ac=c Then, b b can be anything, giving 11 possibilities for b b .

If c = 0 c=0 , then a a can be anything, giving 11 possibilities. If c 0 c\neq 0 , then a = 1 a=1 , and c c can be anything except 0, giving 10 possibilities.

The answer is 11 ( 11 + 10 ) = 231 11(11+10)=\boxed{231} .

Can you elaborate your logic?

Ramakrishna Salagrama - 7 years, 6 months ago

I mean the last step 11 (11 + 10) = 231. How you got?

Ramakrishna Salagrama - 7 years, 6 months ago

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b b can be anything, giving the first 11.

Cases for a a and c c are either c = 0 c=0 or a = 1 a=1 or both. The number of ways this can happen is 11+11 (11 for each) -1 (we overcount if both).

Daniel Chiu - 7 years, 6 months ago
Raj Magesh
Nov 25, 2013

We have: a(b+c) = ab +c

Expanding, ab + ac = ab + c

ac = c

This only occurs when a = 1, or c = 0.

If a = 1, b and c both range from 0 to 10 (11 possibilities each), giving us 11 x 11 = 121 ordered triples.

If c = 0, a and b both range from 0 to 10 (11 possibilities each), giving us 11 x 11 = 121 ordered triples.

Now, all that is left is to check which triples are repeated between the two sets. Obviously, the repeated triples will be the ones where a = 1 and c = 0. Hence, b ranges from 0 to 10 (11 possibilities).

The total number of ordered triples will be 121 + 121 - 11 = 231.

Juss Lunz
Nov 25, 2013

The equation is true when a=1 or c=0

for a=1 -- P(a), we have 11x11 = 121 possibilities (11 from b and 11 from c)

for c=0 -- P(c), we have 11x11 = 121 possibilities (11 from a and 11 from b)

for a=1 AND c=0 -- P(a and c), we have 11 possibilities

So for a=1 OR c=0 -- P(a or c) = P(a) + P(c) - P(a and c) = 121 + 121 -11 = 231

Anis Abboud
Nov 24, 2013
  • a ( b + c ) = a b + c a b + a c = a b + c a c = c a = 1 a(b+c) = ab + c \Rightarrow ab + ac = ab + c \Rightarrow ac = c \Rightarrow a = 1 or c = 0 c = 0 .
  • There are 1 11 11 triples where a = 1 a = 1 , and 11 11 1 triples where c = 1 c = 1 .
  • Note that if we simply sum up the two numbers above, we'll be counting instances where both a = 1 a = 1 and c = 0 c = 0 hold twice. There are 1 11 1 such instances, so we should subtract them once.
  • 121 + 121 11 = 231 \Rightarrow 121+121-11 = \boxed{231} .
Linus Setiabrata
Nov 24, 2013

Distributing a ( b + c ) = a b + c a(b+c)=ab+c gives us a b + a c = a b + c ab+ac=ab+c ,

or a c = c ac=c ,

so a=1 or c=0

for all c=0 we have 121 ordered pairs from (0,0,0) to (0,10,0) (11 ordered pairs for each value of a, and we have 11 values from 0 to 10 inclusive)

for all a=1, c=/=0 we have another 110 ordered pairs from (1,0,1) to (1,10,1) (11 ordered pairs for each value of b, and we have 10 values from 1 to 10 inclusive)

Therefore our final answer is 121 + 110 = 231 121+110=\boxed{231}

Felipe Guima
Nov 24, 2013

a × ( b + c ) = a × b + a × c = a × b + c a × c = c a\times (b+c)=a\times b+ a\times c=a\times b+c \therefore a\times c=c If c 0 a = 1 (I) c \neq 0 \Rightarrow a=1 \text{(I)} If c = 0 a can be anything! (II) c=0 \Rightarrow a \text{ can be anything! (II)}

(I) 1 × 11 × 10 = 110 1\times 11 \times 10 = 110 (II) 11 × 11 × 1 = 121 110 + 121 = 231 11 \times 11 \times 1 = 121 \rightarrow 110+121=231

Let's first consider these sets : X = { 0 , 1 , 2 , 3 , , 10 } , Y = { 1 , 2 , 3 , , 10 } X = \{ 0 , 1 , 2 , 3 , \dots , 10 \} \ \ \ \ , \ \ \ \ Y = \{ 1 , 2 , 3 , \dots , 10 \} Now : a × ( b + c ) = a × b + a × c \because a \times (b+c) = a \times b + a \times c according to the right distributive property in multiplication , and a × ( b + c ) = a × b + c \because a \times ( b + c) = a \times b + c (given) . a × b + a × c = a × b + c \therefore a \times b + a \times c = a \times b + c Then by subtracting ( a × b ) (a \times b) from both sides (there is no problem in that) , we get : a × c = c \large a \times c = c Now let's assume that c 0 c \neq 0 that would be much easier , isn't it ?

Case 1 : ( c 0 c \neq 0 ) : a = c c = 1 \ \ \therefore \ a = \frac{c}{c} = 1 , in this case we have a = 1 , b X a = 1 \ , \ b \in X but c Y c \in Y But what if c = 0 c = 0 ?

Case 2 : ( c = 0 c = 0 ) : By substituting c c we get : a × 0 = 0 a \times 0 = 0 and this is satisfying , in this case we have a , b X , c = 0 a , b \in X \ \ , \ c = 0 .

Now in case 1 as we have a = 1 a=1 fixed in the ordered triple ( a , b , c ) = ( 1 , b , c ) (a,b,c) = (1,b,c) so we look for the ordered pairs ( b , c ) (b,c) , so we get the product set of X × Y X \times Y which contains 11 × 10 = 110 11 \times 10 = 110 possible ordered pairs for ( b , c ) (b,c) and putting 1 1 at the first of each ordered pair we have 110 110 ordered triples .

In case 2 the same but we have c = 0 c=0 fixed in the ordered triple ( a , b , c ) = ( a , b , 0 ) (a,b,c) = (a,b,0) so we look for the ordered pairs ( a , b ) (a,b) , so we get the product set of X × X X \times X which contains 11 × 11 = 121 11 \times 11 = 121 possible ordered pairs for ( a , b ) (a,b) and putting 0 0 at the end of each ordered pair we have 121 121 ordered triples .

Summing all triples up 110 + 121 = 231 \Rightarrow \ 110 + 121 = \boxed{231} ordered triples .

:) Yay!

Calvin Lin Staff - 5 years, 10 months ago

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Pleased to see that you liked it ^_^ .

Mohamed Ahmed Abd El-Fattah - 5 years, 10 months ago

Isn't this much of an over-rated problem ? Simple casework, I guess level 3.

Venkata Karthik Bandaru - 5 years, 9 months ago
Benjamin Kan
Jan 17, 2014

We can simplify a ( b + c ) = a b + c a\cdot(b+c)=a\cdot b+c into a b + a c = a b + c a\cdot b + a \cdot c=a\cdot b+c which is further simplified to a c = c a \cdot c=c This will only be true if a = 1 a=1 , or c = 0 c=0 , or both. Let's count the cases:

Case 1 \text{Case} 1 : Only a = 1 a=1 . Then, for the ordered triple a , b , c a, b, c , we have 1 1 choice for a a , (it can only be 1 1 ), 11 11 choices for b b , (it can be any integer between 0 0 and 11 11 ) and 10 10 choices for c c (it can be any integer between 1 1 and 10 10 ; just not 0 0 ). 1 11 10 = 110 1\cdot 11 \cdot 10=\boxed {110}

Case 2 \text{Case} 2 : Only c = 0 c=0 . Then for the ordered triple a , b , c a, b, c , we have 10 10 choices for a a , (it can be any integer between 0 0 and 10 10 ; just not 1 1 ), 11 11 choices for b b , (it can be any integer between 0 0 and 11 11 ) and 1 1 choice for c c (it can be 0 0 ). 10 11 1 = 110 10 \cdot 11 \cdot 1=\boxed {110}

Case 3 \text{Case} 3 : Both a = 1 a=1 and c = 0 c=0 . Then for the ordered triple a , b , c a, b, c , we have 1 1 choice for a a , (it can only be 1 1 ), 11 11 choices for b b , (it can be any integer between 0 0 and 11 11 ) and 1 1 choice for c c (it can be 0 0 ). 1 11 1 = 11 1 \cdot 11 \cdot 1=\boxed {11}

Adding these 3 3 numbers together, we have 110 + 110 + 11 = 231 110+110+11=\boxed {231} different ordered triples of ( a , b , c (a, b, c that satisfy a ( b + c ) = a b + c a\cdot(b+c)=a\cdot b+c if ( a , b , (a, b, and c c are all between 0 0 and 10 10 inclusive.

Nahom Yemane
Dec 28, 2013

So expanding you have
a b + a c = a b + c ab+ac=ab+c

You can cancel a b ab out to get

a c = c ac=c

From here you have 2 paths- either a = 1 a=1 or c = 0 c=0 and a a can be any value. After this it basically becomes a Combinatorics problem.

When c = 0 c=0 you can 11 11 = 121 11*11=121 ways of picking a a and b b since we have 0 a , b , c 10 0 \leq a,b,c \leq 10

When a = 1 , c 0 a=1, c \not=0 you have 10 values of c c to choose from since it cant be 0 and 11 values of b b to choose from

Which gives 10 11 = 110 10*11=110 ways of picking c c and b b

So the total number of triples is

121 + 110 = 231 121+ 110= \boxed{231}

This post realizes me. Since I neglect the possibility that a = 1 a=1 :(

Muh. Amin Widyatama - 7 years, 5 months ago
Shubham Kumar
Nov 27, 2013

We can write a C++ Coding as follows;

include<constream.h>

void main()

{ clrscr();

int i,j,k,count=0,a,b;

for(i=0;i<=10;i++)

{ for(j=0;j<=10;j++)

{ for(k=0;k<=10;k++)

  {

   a = i*(j+k);

   b = i*j+k;

   if(a==b)

count++;

  }

}

}

cout<<"\n\n\tAns = "<<count;

getch();

return;

}

Output: Ans = 231

Adel Ali
Nov 27, 2013

Lets write the two expressions after distributing a a .

Then we have:
a × b + a × c = = a × b + c a \times b + a \times c == a \times b + c

Obviously there are only two cases when the two equations are equal:

  1. When a = = 1 a == 1

  2. When c = = 0 c == 0

a = = 1 a == 1 O R OR c = = 0 c == 0

When either of the two conditions are satisfied, both equations will be equal. Then for the 1st condition we have 11 × 11 11 \times 11 possibilities, and for the 2nd one we also have 11 × 11 11\times 11 possibilities. Now some triplets have been counted twice and those are when c = 0 c = 0 A N D AND a = 1 a = 1 . By inclusion and exclusion principle, we need to subtract these triplets from our answer, so the final answer is ( 11 × 11 ) + ( 11 × 11 ) ( 11 ) = 231 (11 \times 11) + (11 \times 11) - (11) = \boxed{231}

Taisuke Yasuda
Nov 25, 2013

If we distribute a(b+c), we get ab+ac = ab + c. Then, we can subtract ab from both sides, ending with ac = c. Ordered pairs that satisfy this are ordered pairs with a = 1 or c = 0. Now we can do casework, counting the number of ordered pairs with (1,b,c) or (a,b,0). We have 11 possibilities for each letter, giving us 121 possibilities a = 1 and 121 for c = 1. However, we count any ordered pair (1,b,0) twice, which has 11 possibilities. Therefore, the grand total is 121+121-11=231.

Marco Massa
Nov 25, 2013

If a x (b+c) = a x b + c, then a x c = c , then a = 1, * if c /= 0 * . We have: * if c /= 0 * the ordered triples are (1,b,c) with *c/= 0 * and so we have 11x10 = 110 triplets. If c = 0 we have axb = axb that is true for all a,b, so we have 11x11= 121 triplets like (a,b,0). In total we have 110+121=231 triplets.

Lu Chee Ket
Sep 29, 2015

Computer is always reliable for this kind of question. Answer: 231

Ali Qureshi
Aug 2, 2015

we have two cases case 1. (when a=1) the possible number of triplets are 1 * 11 * 11=121 case 2. (when c=0) the possible number of triplets are ( 11 * 11 * 1)-(1 * 11 * 1)=110 we subtract because it has occurred in case 1. so the possible number of triplets which satisfy the property are 121+110=231

Did the same way !!!

Nam Diện Lĩnh
Jun 16, 2015

counting using javascript

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var count=0;
for(var a=0;a<11;a++)
    for(var b=0;b<11;b++)
        for(var c=0;c<11;c++)
            if(a*(b+c)==a*b+c)
                count++;

console.log(count);

By expanding the equation, we get:

a × b + a × c = a × b + c a \times b + a \times c = a \times b + c

a × c = c ( 1 ) a \times c = c (1)

Now, the original equation will be true when (1) is true.

Since b is eliminated completely from the result equation, b can take any value in range [0,10] without affecting the validity of (1). There are 11 possibilities for b.

For (1) to be true, either:

  • a = c c = 1 a = \frac{c}{c} = 1 c can not be 0

a can only be 1. c can take any value in range (0,10], thus has 10 possibilities.

The number of combinations of a and c in this case is 1 × 10 = 10 1 \times 10 = 10

  • c = 0 c = 0

a can take any value in range [0,10], thus has 11 possibilities. c can only be 0.

The number of combinations of a and c this time is 1 × 11 = 11 1 \times 11 = 11

The number of total possible combination of a and c is 11 + 10 = 21.

The number of combinations of a,b and c are 11 × 21 = 231 11 \times 21 = 231

Mohamed Mahmoud
Dec 6, 2013

that equation is correct if a=1, or c=0

number of orders with (a=1) = 11*11=121, as we have 11 numbers from 0 to 11 could be the values of b and c

number of orders with (c=0) = 11*11=121, as we have 11 numbers from 0 to 11 could be the values of a and b

number of orders with (a=1) and (c=0) = 11, as we have 11 numbers from 0 to 11 could be the values of b

number of orders with (a=1) or (c=0) =number of orders with (a=1)+number of orders with (c=0)-number of orders with (a=1) and (c=0)=121+121-11= 231 \boxed{231}

Gautam R
Dec 3, 2013

a(c-1)=0

Possibilities:

if a=1 then c can be from 0 to 10. if c=1 then a can be from 1 to 10. so totally 21 possibilities. b can be from 0 to 10, so 11 posibilities. 21*11=231

Avijeet Ghosh
Dec 3, 2013

(axb)+(axc)=(axb)+c =>ac=c...this means it does not depend on b....so every 11 values of b is possible for each duplet (a,c)...so a factor of 11 is included to the number of triplets... now, for c not equal to 0,a=1 therefore, all duplets (a,c)= (1,1),(1,2)...(1,10)...that is 10 duplets are possible.... again,if c=0,all 11 values of a is valid...therefore 11 duplets so total number is=11x(11+10)=231

Samanyu Kansara
Nov 25, 2013

When c = 0, a and b both can take 11 values (0..10 inclusive) that gives us 11 x 11 possible triplets. When c = (1..10), a has to be 1 and b can still take 11 values. That gives us another 11 x 10 possible triplets. Add 121 + 110. That is 231.

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