arithmetic
posted by Cennie .
In an arithmetic series, the terms of the series are equally spread out. For example, in
1 + 5 + 9 + 13 + 17, consecutive terms are 4 apart. If the first term in an arithmetic series is
3, the last term is 136, and the sum is 1,390, what are the first 3 terms?

have you learned this property ?
S(n) = (n/2)(first + last) ?
1390 = (n/2)(3 + 136)
I get n = 20
so term(20) = a+19d = 136
19d = 133
d = 7
term 1 = 3
term 2 = 10
term 3 = 17