Which of the following numbers is divisible by 3, 7, 9 and 11 ?

Clearly, 639 is not divisible by 7
Consider 2079
Sum of its digits
= (2 + 0 + 7 + 9)
= 18
So, it is divisible by both 3 and 9
Also, (79 - 2) = 77, which is divisible by 7
So, 2079 is divisible by 7
Also, (9 + 0) - (7 + 2) = 0
So, 2079 is divisible by 11
Hence, 2079 is divisible by each one of 3, 7, 9 and 11