Q1. If is a multiple of 9, where y is a digit, what is the value of ?
If a number is a multiple of 9 , then the sum of its digit will be divisible by 9.
Sum of digits of 21y5= 2 + 1 + y + 5 = 8 + y
8 + y is a multiple of 9 when y = 1 (since y can only be single digit )
hence y = 1 is correct answer.