Very easy question, please help!

Hello, I have a simple question (I hope!) with regard to a concept in investment management called "Maintenance Margin" I won't explain the theory, I just need to know how the equation is computed. Here it is:

(100P-4,000) / 100P = .30

The P will represent the stock price in which to make a margin call, and the answer should be $57.14 I just don't understand how the problem was computed. I tried multiplying both sides of the equation by 100P, to get rid of the fraction, but this does not seem to work. Please help, thanks!