Question

You are taking a test that contains 60 math problems. There is a time limit of 90 minutes in which to complete it. After doing 30 problems, you look at the clock and realize you must work twice as fast in order to complete the test in the allotted time. How many problems must you do per minute on the second half of the test in order to finish on time? Use the following equation:
30/x+30/2x=90