One common factor of two numbers is 40. another common factor is 10. if both numbers are less than 100, what are the two numbers?

Only two numbers less than 100 have a factor of 40: 40 (1 x 40) and 80 (2 x 40). Since 40 is a factor of both 40 and 80, 10 must also be a factor - indeed, 40 = 4 x 10 and 80 = 8 x 10.

The two number must be 40 and 80.