39==3/2*[20 + 2D], solve for D
178==4/2 * [2F + 3*3 ], solve for F
D ==3 - this is the common difference
F ==40 - this is the first term of the last 4 terms.
Sequence: 10, 13, 16.............40, 43, 46, 49...
Your full sequence looks like this:
10 , 13 , 16 , 19 , 22 , 25 , 28 , 31 , 34 , 37 , 40 , 43 , 46 , 49 , >>Number of terms = 14>>Total Sum == 413
The average of all 14 terms==413 / 14 ==29.5