Question

If 21y5 is a multiple of 9, where y is a digit, which digits can take the possible value of y?

Answer 1

Since 21y5 is a multiple of 9, therefore, according to the divisibility rule of 9, the sum of all the digits should be a multiple of 9.

2 + 1 + y + 5 = 8 + y should be divisible by 9.

This is possible when 8 + y is any one of these numbers 0, 9, 18, 27, and so on …….

However, since y is a single digit number, this sum can be 9 only.

Therefore, 8 + y = 9, giving y = 1