Answer:
20
Step-by-step explanation:
Sn = (n/2)(a1 + a1 + d(n-1)), where S = sum, a1 = first term, n = number of terms, d = common difference
400 = (n/2)(1 + 1 + 2(n - 1))
400 = (n/2)(2 + 2n - 2)
400 = (n/2)(2n
400 = n2
20 = n
Twenty numbers are in the set.
