Question

one common factor of 2 numbers is 40 another common factor is 10 if both numbers are less than 100 what are the two numbers

Answer 1

so, if one common factor is 40, then they also have all the factors of 40 in common: 2,4,5,8,10... so we don't need to know that the other common factor is 10. 

but there are only two multiples of 40 smaller than 100: 40 and 80. and if those numbers are not the same number, then they must be 40 and 80!