For what value of digit x is :

31x5 divisible by 3 ?

According to the divisibility of 3, if the sum of the digits of the number is divisible by 3 then the number is divisible by 3.

31x5 = 3 + 1 + x + 5 = 9 + x. And (9 + x) should be a multiple of 3.

Hence, 9 + x = 0 ,3, 6, 9, 12, 15, 18, ……

x = -9, -6, -3, 0, 3, 6, 9,…..

Since, x is a digit. ∴ x = 0, 3, 6 or 9