In Chemistry class, the ratio of passing grades to failing grades is 7 to 5. How many of the 36 students failed the chemistry class?

Passing to failing ... ratio 7:5 ... added = 12

so 7/12 of the total students are passing and 5/12 of the total students are failing. There are 36 total students.

7/12 (36) = 252/12 = 21 are passing

5/12 (36) = 180/12 = 15 are failing <==