Can you please explain this to me.

The problem is 7/16 - 3/20

First I have to find the common denominator which would be 80

How does the 7/16 become 35/80 (where does the 35 come from)
How does the 3/20 become a 12/80 (Where does the 12 come from)

35/80 minus 12/80

answer 23/80

3 answers

7/16*5/5= 35/80

3/20 * 4/4= 12/80
I still don't get it???
What you don't understand it how to get a common denominator...

http://www.mathsisfun.com/numbers/common-denominator.html