Question

What is the greatest common factor of 30, 45, and 75?

Answer 1

First, write out all the prime factorisation of 30, 45, and 70. 
30: 2*3*5
45: 3*3*5
70: 2*5*7
Then, find a number that all three numbers have in common; which is 5. So, 5 is the greatest common factors of 30, 45, and 70.