The mystery number is 5.
The mystery number is a factor of 40 so the first thing to do is to list the factors of 40:
The number also has to be a factor of 15 because only a number that is a factor of 15 can have 15 as a multiple.
Factors of 15 are:
The only number appearing on both lists is 5 so we can conclude that 5 must be the mystery number.
Find out more at https://brainly.com/question/20514664.