Sam has 1500 marbles and he puts them into a box with 40 holes. He put 1 marble in the first hole. 2 marbles in the second hole. 3 marbles in the third hole, and so on until all 40 holes are filled with marbles. How many marbles are left when he has filled all the holes?
1+2+3+... for 40 terms
a=1, d=1, n=40
sum(40) = 20(2 + 39(1)) = 820
number left = 1500-820 = ....