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