College Compass keeps students familiar with the different types of questions they will face on Test-Day. By following our weekly Example Problem blog posting you will be more prepared than ever to solve PSAT/SAT questions.
Consider this PSAT/SAT Math problem:
- Once a plant starts to grow, it doubles its height by the end of each day. It is 200 millimeters tall at the end of its seventh day. In millimeters, how much taller is it at the end of the eleventh day than at the end of the seventh?
Explanation: First, forget about the word millimeters; for this question, you are not going to have to make any kind of conversion. The term has been put in there just to confuse you.
Next, we know we have a plant that doubles in height every day. We also know that we are being asked to figure out how much taller the plant will be after four days (days 8, 9, 10, and 11). Note: We are NOT being asked to figure out how tall the plant will be after four days, but how much taller it will be after four days.
We are starting with a height of 200. This height will double in length every day for four days (Days 8, 9, 10, and 11). So:
Day 8 = 200 + 200 = 400
Day 9 = 400 + 400 = 800
Day 10 = 800 + 800 = 1,600
Day 11 = 1600 + 1600 = 3,200
So, at the end of the eleventh day the plant will have reached a height of 3,200. Remember though, you are being asked to determine how much taller the plant is on the eleventh day than it was on the seventh day.
3,200 – 200 = 3,000
The answer is (a).
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