A Triple Prime

The three digit number X Y Z \overline{XYZ} is such that X , Y X ,Y and Z Z are different primes and the number is divisible by each of them. What is the number?


The answer is 735.

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4 solutions

Sean Sullivan
Jul 28, 2015

Since 0 < X , Y , Z < 10 0< X, Y, Z<10 and all are prime we have X , Y , Z { 2 , 3 , 5 , 7 } X,Y,Z\in\{2,3,5,7\}

If any two are 2 2 and 5 5 the last digit of X Y Z \overline{XYZ} is 0 0 which is not possible value.

We have ( X , Y , Z ) (X,Y,Z) is a permutation of 2 , 3 , 7 2,3,7 or 3 , 5 , 7 3,5,7 , by divisibility rules for 2 2 and 5 5 each of these cases has Z = 2 Z=2 and Z = 5 Z=5 respectively.

All permutations are already divisible by 3 3 becasue 2 + 3 + 7 = 12 2+3+7=12 and 3 + 5 + 7 = 15 3+5+7=15 are both mutliples of 3 3

Divisibility rule for 7 7 says subtract 2 2 times the last digit from the others

For Z = 2 Z=2 we have 37 4 = 33 37-4=33 and 73 4 = 69 73-4=69 , neither are divisble by 7 7

For Z = 5 Z=5 we have 37 10 = 27 37-10=27 and 73 10 = 63 73-10=63 and 7 63 7\mid63

So we have X Y = 73 \overline{XY}=73 and subsequently X Y Z = 735 \overline{XYZ}=\boxed{735}

Prakash Arora
Aug 22, 2020

Short Python Solution. Directly yield output : 735

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def prime(i):
    if i<2:
        return False
    for j in range(2,i):
        if i%j==0:
            return False
    return True
for i in range (100,1001):
    j=i
    a=j%10
    j//=10
    b=j%10
    j//=10
    c=j
    if prime(a) and prime(b) and prime(c) and i%a==0 and i%b==0 and i%c==0 and a!=b and b!=c:
        print(i)

Output:
735

Emmanuel Lopez
Oct 9, 2015

There are 4 primes to use for XYZ

2357

The number has to be divisible by 3 of them, which means it has them as factors

210=2 3 5*7

210/2 = 105 as base? maybe

210/3 = 70 as base? no, zero rules it out

210/5 = 42 as base? maybe

210/7 = 30 as base? no zero rules it out.

Take the numbers 2 & 5...

multiples of 5 end in either 5 or 0, only 5 is prime

multiples of 2 can be 2 4 6 8 and 0, only 2 is prime

This means that it has to end with either 2 or 5, but with 210*N added on to it.

42? no

252? no

462? no

672? no

882? no

1092? no

105? no

315? no

525? no

735? yes! <---

945? no

Curtis Clement
Jul 28, 2015

There might be a neat way of doing this but I am going to go for a longer, more accessible method called casework.It is important to note that the primes available are 2,3,5 and 7. Now lets start by looking at the last digit Z. If Z=2, then we have 6 possibilities: (X,Y) = (3,5) , (3,7) , (5,7) + reverse order. However, if X or Y equals 3 then the digit sum must be divisible by 3 (divisibility by 3 rule) so (3,5) & (5,3) aren't valid. Also, X or Y can't equal 5 otherwise Z= 5, which contradicts Z=2. This excludes all possibilities for Z=2.

For Z=5, we have (X,Y) = (2,3) , (2,7) , (3,7) +reverse. Now using the divisibility by 3 rule (3,7) & (7,3) are the only possibilities left. Testing both gives XYZ = 735 \boxed{735} = 3 × 5 × 7 2 \times\ 5 \times\ 7^{2} . Now we just need to verify that there are no more solutions.

For Z=3: we use the divisibility by 3 rule again to exclude (2,5) & (5,2). Now if 5 is a digit then 5| XYZ but that implies that Z=5 so (5,7) &(7,5) are also excluded. (2,7) and (7,2) are excluded as well as XYZ is odd. So no solutions for Z = 3.

For Z=7: (2,5), (3,5) + reverse are excluded as 5 doesn't divide XYZ unless Z=5. Similarly (2,3) +reverse is excluded as 2 can't divide an odd number.

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