If 21y5 is a multiple of 9, where y is a digit, which of the numbers can take the possible value of y?
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.
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 = 1