PSAT Math Grid-Ins Question 138: Answer and Explanation

Question: 138

Eloise is told by her doctor that she should try to average 9 hours of sleep a night because that is what a typical teenager needs for optimal mental and physical health. If Eloise was awake for 126 hours in a given week, how many additional hours of sleep should she have had in order to follow her doctor's advice?

Correct Answer: 21

Explanation:

21 In a typical week, Eloise should get 9 × 7 = 63 hours of sleep. To see how many hours she has actually slept, subtract the total hours she has been awake from the total hours in a week:

(24 × 7) - 126 = 42

Then calculate the additional hours of sleep she should get by subtracting how many hours she actually got (42) from the amount of sleep she should have gotten (63):

Eloise should get an additional 63 - 42 = 21 hours of sleep.