If 21y5 is a multiple of 9, where y is a digit, what is the value of y?


We know that. if a number is a multiple of 9, then the sum of its digits will be divisible by 9


Sum of digits of 21y5 = 2 + 1 + y + 5 = 8 + y


Hence, 8 + y 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 should be 1 only


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