Set up a proportion with the information you have, like so,
[tex] \frac{75}{x} = \frac{30}{100} [/tex]
Cross multiply like so,
[tex]30x = 7500[/tex]
Divide by 30 to isolate x, and you will get 250 as x.
So, to answer your question, subract 75 from 250 and it will give you how much money is left to pay (175)
