To get the index form, you basically divide the given number by factors from smallest to largest until you are left with 1. So we start off with the smallest divisible number, 2. If you refer to my workings at the side, I divide the original number by 2 continuously, until it is no longer possible to get a whole number from dividing by 2. Then you will move on to the next smallest number that the number can be divided by. So using the example for 20, 20/2 = 10, 10/2=5, and since 5 cannot be divided by 2 anymore to give us a whole number, we will look for another divisible number to use, which in this case only 5 is possible. Hope this helps!