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

Property:
If a number should be divisible by nine, then the sum of the digits must be divisible by nine.

According to the property:
2 + 1 + y + 5 = Multiples of nine
=> 8 + y = 9 (suppose, because y must be 1 digit)
therefore y= 1 only.

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