First, set up a ratio: 
The first fraction is the percentage (30% and
are the same thing) and the second is the loan payment (which is equal to the 30 in the first fraction) over their annual income (
).
First you divide $15,680 by 30, then multiply that quantity by 100. The value calculated then replaces in your second fraction: 
The first fraction is the percentage (30% and
First you divide $15,680 by 30, then multiply that quantity by 100. The value calculated then replaces