First, set up a ratio: [tex] \frac{30}{100} = \frac{15,680}{x} [/tex]
The first fraction is the percentage (30% and [tex] \frac{30}{100} [/tex] 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 ([tex]x[/tex]).
First you divide $15,680 by 30, then multiply that quantity by 100. The value calculated then replaces [tex]x[/tex] in your second fraction: [tex] \frac{30}{100} = \frac{15,680}{52,266.67} [/tex]
