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Advanced MathEasy–MediumExtra customers between two days of doubling

Advanced Math practice question

In its first week, a new shop's number of customers doubled every day: 5 on day 1, 10 on day 2, and so on. How many more customers came on day 4 than on day 2?

Answer: 30

Day 2 had 10 and day 4 had 40, so the difference is 30.

Common wrong answers

If you answered 10
10 is the difference in days times 5, treating the growth as linear.
If you answered 20
20 doubles day 2 without subtracting.
If you answered 40
40 is day 4 alone.

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