Question 336210
Did you try just adding them up?. 20+21+22+23+24+....+39

There is a shortcut, though, using the sum of arithmetic sequences.

{{{Sn=n/2*(a1+a2)}}}

{{{a1=20}}}

{{{an=39}}}

{{{d=20}}}

{{{20/2*(20+39)=10(59)=590}}}