A mystery number is a factor of 100 and a common multiple

of 2 and 5. The sum of its digits is 5. What is the number?

50

Step-by-step explanation:

The number is a common multiple of 2 and 5, which means it is a multiple of 10. So the last digit is 0.

The sum of the digits is 5. The number is a factor of 100, so the only possible solution is 50.