There are some biscuits in a box. The biscuits can be put into plates of 5 or 15 with no biscuits leftover. When the biscuits are put into plates of 7, there are short of 5 biscuits on the last plate. If there are more than 30 biscuits but less than 50 biscuits, how many biscuits are there in the box?
If biscuits can be put into plates of 5 or 15 with no biscuits leftover, the number of biscuits is a multiple of 15 as 15 is a multiple of 5.

Multiples of 15 that are greater than 30 but less than 50: 30, 45

Multiples of 7 that are greater than 30 but less than 50: 35, 42, 49
Multiples of 7 (- 5) that are greater than 30 but less than 50: 30, 37, 44

Common number = 30

Number of biscuits = 30

Answer(s): 30