Answer:
The number must be 30.
Step-by-step explanation:
The mystery number is less then 40.
You say its name when you start at 3 and count by 3s. This means that it is a multiple of 3.
You say its name when you start at 5 and count by 5s. This means that it is a multiple of 5.
The only two numbers that are multiples of 3 and 5 and less than 40 are 15 and 30.
Since we have been told that the number is not 15, the number must be 30.
