Solution:

Provided that,

The first term, a = 5

The last term, l = 45

The sum of the AP be, S_n = 400

The sum of the AP formula, as we all know, is;

S_n = n/2 (a+l)

400 = n/2(5+45)

400 = n/2(50)

The number of terms, n =16

As we all know, the AP series’ last term can be written as;

l = a+(n −1)d

45 = 5 +(16 −1)d

40 = 15d

The common difference, d = 40/15 = 8/3