Which number fits next in the series 2, 3, 10, 15, 26, ?

Explanation

1. 01

   Compare each term with a square: $2 = 1^2 + 1$, $3 = 2^2 - 1$, $10 = 3^2 + 1$, $15 = 4^2 - 1$, $26 = 5^2 + 1$.

2. 02

   The +1 and −1 alternate, so the 6th term is $6^2 - 1 = 35$.