The factors of 72 are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36 and 72. Factors are the numbers that can be multiplied together to get 72.
First, all numbers are divisible by themselves and 1. If a number is even, one of the factors is 2, and another is whatever 2 needs to multiply with to get the number. Then, If the sum of the digits (in this case, 7 + 2 = 9) is divisible by 3, one of the factors is 3, and another is whatever 3 needs to be multiplied by to get the number.
After that, try dividing by four. If the product is a whole number, then 4 is one of the factors along with whatever 4 needs to multiply with to get the number. The same process can be followed with the rest of the numbers 5 through 10. If the answer is a whole number, two factors have been identified.