A classic MATHCOUNTS problem

How many integers n n satisfy the condition 100 < n < 200 100 < n < 200 and the condition n n has the same remainder whether it is divided by 6 or by 8?


The answer is 25.

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4 solutions

First we find LCM of 6 and 8, which is 24. Then we find the factors of 24 in the specified range. 96 Is close but not in, so then we get 120, 144, 168, and 192, which is 4 solutions so far. These all have remainder 0 when divided by either number. But what about other remainders? We notice that every number after the multiples of 24 is a solution if it is less than the multiple plus 6, because the remainder is anywhere from 0 - 5. So this is 6 in total for each multiple, so 6 * 4 is 24. But there's one more. Back to 96. The first one above 100 and last one before 102, is a solution. So the answer is 25 \boxed{25}

:D Perfect!

Finn Hulse - 7 years, 3 months ago

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:( darn, I thought that 100 n 200 100 \le n \le 200 , so I answered 26 26 at first . _ __ .

Michael Diao - 7 years, 1 month ago

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Wait... don't you mean 24?

Finn Hulse - 7 years, 1 month ago

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@Finn Hulse i just skipped 101 and got 24

akash deep - 6 years, 9 months ago

that is it

Mohamed Elzoghpy - 7 years, 3 months ago

Main strategy is you have to count for those numbers which is divisible by 6 and 4

Tahsin Ahmed - 7 years, 1 month ago
Vighnesh Raut
Mar 12, 2014

include <iostream.h>

 #include   <conio.h>
 main()

{ clrscr(); int i,j=0; for(i=101;i<200;i++) if(i%6==i%8) j++; cout<<j; getch(); return 0; }

Okay... ?

Finn Hulse - 7 years, 3 months ago

that's not that difficult dude ! you don't need computer for that !!!

Priyansh Saxena - 7 years, 3 months ago

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but still , it is fun making programs....

Vighnesh Raut - 7 years, 2 months ago
Aditya Joshi
Mar 7, 2014

Remainder when 101 101 is divided by 8 8 is equal to when 101 101 is divided by 6 6 . The next time the remainder is same is after 18 18 numbers. If we check the remainders for 100 , 99 , 98 , 100,99,98, \dots , they are 4 , 3 , 2 , 0 4,3,2, \dots 0 respectively. We notice that the remainders are the same after a period of 18 18 numbers.

So, 18 18 numbers after 101 101 , as we observed, the remainders are synchronized, thus, at number 102 + 18 = 120 102 + 18 = 120 . Now, we have 6 6 numbers of common remainders, 0 , 1 , 2 , 3 , 4 , 5 0,1,2,3,4,5 . Thus, we have come up to 120 + 6 = 126 120 + 6 = 126 . Now, the remainders will synchronize again in 18 18 steps, thus, at 126 + 18 = 144 126 + 18 = 144 . Now, there will be 6 6 numbers of the same remainder. In short, every 24 24 numbers, we have 6 6 numbers with common remainders. Thus, above 101 101 , there are a total of 4 4 multiples of 24 24 that fit our criteria and the first number 101 101 . Thus, we have a total of 1 + 6 + 6 + 6 + 6 = 25 1 + 6 + 6 + 6 + 6 = \boxed{25} numbers.

Aman Kumar
Mar 19, 2014

LCM of 6 and 8 is 24.........choose the multiple of 24 between 100 and 200 where 100 and 200 are not included..... the numbers are 120,144,168,192.........for each cases there are up to (number+5) have same reminder....so total number of cases are......... .multiplication.(4 ,6)=24..........here we r forgetting a number 101 i.e............(multiplication(24*4)+5)...............so total is (24+1)=25...........

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