You have to find the first and the last number in range that is divisible by 6,
So the numbers are 24 and 78
Now use A.P
78 = 24 + (n-1) *6
=> (78-24)/6 = n-1
=>54/6 = n-1
=> n = 9 +1
=> n = 10
Answer is 10.
and numbers are 24,30,36 42,48,54,60,66, 72 and 78