200 * 2/15 = the number of students that didn't read any books. This is 26.6, so 40 is not a good estimate 40 students is 20 percent versus 26 being approximately 13 percent.
You just have to compare the ratio you're given to the answers and see if the ratios from the answers match up to the ratio you were given. The reason C is the answer is because if you divide by 3 in the ration, it is the same as the ratio you were given, 7:3
