Question

Which is not a common factor of 15,45,90?

Answer 1

Factors of 15 are 1,3,5 and 15
Factors of 45 are 1,3,5,9,15,45
Factors of 90 are 1,2,3,5,6,9,10,15,18, 30,45,90 
Now just check the numbers between those and you should have your answer