Question

A triangular shaped stack of tin cans has 8 cans in the first row and 8 rows in all. In each successive row one can is removed.

Answer 1

If n is the number of cans in row r
n = 9 - r where r is a natural number less than 9

When r = 5 then n = 9 - 5 = 4

(you could let r = 9 as the ninth row has zero cans but values above 9 will give negative counts)