Just make an A.P. of 6 divisible numbers: but between 20 and 99
Here, Your Series: 24, 30, .... 96
Now, a = 24, d = 30-24 = 6, a(n) = 96
a(n) = a + (n - 1)d
96 = 24 + (n - 1)6
96 - 24 = 6n - 6
72 + 6 = 6n
n = 78/6
n = 13
In short, Your Answer would be 13
Hope this helps!