Which number should replace the question mark in the series 11, 13, 18, 28, 45, ?

The steps between the terms are 2, 5, 10 and 17, which are $1^2+1,2^2+1,3^2+1$ and $4^2+1$. The next step is therefore $5^2+1=26$, and 45 + 26 = 71. Answering 70 comes of adding 25 and forgetting the extra one; 62 of repeating the previous step of 17; 82 of skipping a step and adding $6^2+1=37$.