Which of the following numbers is divisible by 5, 6, and 9? a. 54,183 b. 45,803 c. 81,450 d. 18,045

Straight away it can't be A or B because the last/ones digits aren't multiples of 5.

For something to be divisible by 9, add all the digits up and see if it is a multiple of 9:
This works for both C and D, as both numbers have digits which add up to 18.

However, only C is divisible by 6. D is an odd number and hence it is eliminated. For something to be divisible by 6, it must be divisible by 2 (an even number) and 3 (digits add to a multiple of 3). C does both.
Hence C is the answer.

Hope this helped