Question

If 2791A is divisible by 9 supply the missing digit in place of A

Answer 1

divisibility by 9 is when some of digits is a multiple of 9
2+7+9+1+A=kx9 where k is some constant
19+A=27 as A should be less than 10
A =8

Answer 2

For a number to be divisible by 9 its sum of digits must be divisble by 9
so 
2 + 7 + 9 + 1 + A = 19+A
put A = 8
we get sum as 27 which is divisible by 9
So A = 8
27918 is divisible by 9