On three 150-point geography tests, you earned grades of 88%, 94%, and 90%. The final test is worth 250 points. What percent do you need on the final to earn 93% of the total points on all tests?

You must earn 243 points to have earned 93% of all possible points.

We first multiply each percentage on the previous tests by 150:

0.88*150 = 132
0.94*150 = 141
0.9*150 = 135

The total number of points possible is given by adding up the possible points for the three previous tests and the 250 for the last test:
150+150+150+250 = 700

93% of the 700 points would be 0.93(700) = 651 points.

Now we have 132+141+135+x (last test) = 651
408 + x = 651

Subtract 408 from both sides:
408+x-408 = 651-408
x = 243