Your question transforms to finding the following sum : 3 + 7 + 11 + 15.... upto 15 terms
Use formula for sum of an AP series
S = (n * (2a + (n - 1)d)) / 2
n -> no of terms in AP series , so n = 15
a -> first term in series, so a = 3
d -> common difference, so d = 4
Put all values for a, n, d and you will get the sum.
