There are eight shelves. How many soup cans a grocery clerk stack if the bottom shelf has 21 cans and each additional shelf has 2 fewer soup cans than the previous shelf?

Answer 1

21 + 19 + 17 ...

a (a+d) (a + 2d) ..... a+7d eight terms
where
a = 21
and
d = -2
arithmetic series

sum of first 8 = (8/2) (42 + 7(-2) )
= 4 (42 - 14)
= 4 (28)
= 112