a grocery store display consists of cases of soda. there are 15 cases at the base, & 1case at the top. if there are 15 rows, how many cases are needed for the entire display? a) 180 b)30 c) 130 d) 15
What you have is 1+2+3+...+15
That is just
(1+15) + (2+14) + ...
There are 7 pairs of 16 + 8 in the middle:
112+8 = 120
1+2+...+n = n(n+1)/2 = 15(16)/2 = 120