You have to look for the pattern.  Notice that you add 1,
then you add 2, then you add 3, etc.  See the pattern?
It doesn't take a rocket scientist:

Start with 5.
Add 1 to 5, get 6.
Add 2 to 6, get 8.
Add 3 to 8, get 11.
Add 4 to 11, get 15.
Add 5 to 15, get 20.
Add 6 to 20, get 26.
Add 7 to 26, get 33.   <-- That's the 8th number.
Add 8 to 33, get 41.    
Add 9 to 41, get 50.   <-- That's the 10th number.
Add 10 to 50, get 60.
Add 11 to 60, get 71.
Add 12 to 71, get 83.
Add 13 to 83, get 96.
Add 14 to 96, get 110.
Add 15 to 110, get 125.
Add 16 to 125, get 141.
Add 17 to 141, get 158.
Add 18 to 158, get 176.
Add 19 to 176, get 195.
Add 20 to 195, get 215.


Edwin