Answer:
460
Steps:
The Arithmetic Series formula is:
S = n( (a1 + an) / 2)
Take note that:
S = the sum of n terms
n = number of terms
a1 = the first term
an = the last term
We are given n, a1, an. Let’s solve for S.
S = 20( (18 + 28) / 2)
18 + 28 = 46
46 / 2 = 23
20 * 23 = 460