There are some cookies in a box. The cookies can be put into plates of 5 or 10 with no cookies leftover. When the cookies are put into plates of 13, there are short of 6 cookies on the last plate. If there are more than 20 cookies but less than 40 cookies, how many cookies are there in the box?
If cookies can be put into plates of 5 or 10 with no cookies leftover, the number of cookies is a multiple of 10 as 10 is a multiple of 5.

Multiples of 10 that are greater than 20 but less than 40: 20, 30, 40

Multiples of 13 that are greater than 20 but less than 40: 26, 39
Multiples of 13 (- 6) that are greater than 20 but less than 40: 20, 33

Common number = 20

Number of cookies = 20

Answer(s): 20