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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 21*y*5 = 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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