Which number should come next in the series?

1 - 1 - 2 - 3 - 5 - 8 - 13

This is part of the Fibonacci sequence. The next number is 21. Look at how much each number increases and the number before it.

1+1=2 (the number after 1 and 1)

1+2=3 (the number after 1 and 2)

2+3=5 (the number after 2 and 3)

So to solve we add 13+8=21