question_answer 1)

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

Answer:

                Since \[21y5\] s a multiple of 9, its sum of digits \[2+1+y+5=8+y\] is a multiple of 9; so \[8+y\] is one of these numbers: 0, 9, 18, 27, 36, 45,.... But since y is a digit, it can only be that \[8+y=9\]. Therefore, \[y=1\].