There are some tarts in a box. The tarts can be put into plates of 2 or 6 with no tarts leftover. When the tarts are put into plates of 7, there are short of 5 tarts on the last plate. If there are more than 20 tarts but less than 40 tarts, how many tarts are there in the box?
If tarts can be put into plates of 2 or 6 with no tarts leftover, the number of tarts is a multiple of 6 as 6 is a multiple of 2.

Multiples of 6 that are greater than 20 but less than 40: 24, 30, 36

Multiples of 7 that are greater than 20 but less than 40: 21, 28, 35
Multiples of 7 (- 5) that are greater than 20 but less than 40: 16, 23, 30

Common number = 30

Number of tarts = 30

Answer(s): 30