Algebraic series question

Question

I cannot find the common difference between the series and would appreciate some help solving it thanks.

Find the sum of the following series: 18/12 + 18/20 + 18/30 + 18/42 + ... + 18/1260

Answer 1

18/12 - 18/20 = 3/2 - 9/10= 1.5 =.9 = .6 = 3/5

18/20 - 18/30 = 9/10 - 6/10 = 3/10 = .3

there is no common difference as .6 is half of .3

it's not an Arithmetic series