Which number should come next in the series and how?
1 - 1 - 2 - 3 - 5 - 8 - 13

this series is known as fibinocci series,which means the third no. will be the sum of preceeding nos.
here 1+0=1
        1+1=2
        1+2=3
similarly the next no. after 13 will be the sum of 13 and 8= 21