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.
Therefore, 2+1+y+5=9
=> 8+y = 9
=> y=1
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, y should be 1 only.
