Only 1 goes into 3 and 5, because 3 and 5 are prime numbers. 1 goes into all number. So perhaps the question is: what is the smallest number that 3 and 5 both go into. The answer is 15, but it's called the lowest common multiple (LCM). To find the GCF of two numbers, they must be broken down into their factors first. Then you look at the factors of the numbers to find the largest common factor. So, for example, the GCF of 30 and 50 is 10, because 10 is the largest number that goes into both 30 and 50. 2 and 5 are also common factors, but 10 is the largest.