80, 50 times 9.90 is 495 and 80 times 5.30 is 424, 424 + 495 = 919
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: