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

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!

This equation can be simplified to: x=2 y

y=1/2x

Yes, this is linear