Question

If 21×5 is a multiple of 9, where x is a digit, what is the value of x?​

Answer 1

Answer:

Since the number 21y5 is a multiple of 9.

So, the sum of its digits 2+1+y+5=8+y is a multiple of 9.

∴(8+y) is either 0 or 9 or 18 or ...

Since y is a digit, so (8+y) must be equal to 9.

i.e., 8+y=9

⇒y=9−8=1