A number theory problem by Nikkil V

Find the number of positive integers with three not necessarily distinct digits, a b c abc , with a 0 a\neq0 c 0 c\neq0 .such that both a b c abc and c b a cba are multiples of 4 4 .


The answer is 40.

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1 solution

For a b c abc to be divisible by 4 4 we require only that b c bc be divisible by 4 4 . Allowing for b = 0 b = 0 there are 25 25 such two-digit "constructs", 5 5 of which have c = 0 c = 0 , (namely 00 , 20 , 40 , 60 00, 20, 40, 60 and 80 80 ), leaving us with 20 20 allowable constructs b c bc .

Then for c b a cba to also be divisible by 4 4 we require that b a ba be divisible by 4 4 . For each of the previous 20 20 constructs b c bc there will be 2 2 corresponding constructs b a ba that will be divisible by 4 4 such that a 0 a \ne 0 , (i.e., 2 2 for each value of b b from 0 0 through 9 9 ), giving us a total of 2 × 20 = 40 2 \times 20 = \boxed{40} desired integers.

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