Problem-Solving and Data AnalysisHardNumber of red pens from a probability
Problem-Solving and Data Analysis practice question
A box contains only red pens and blue pens. The probability of drawing a red pen at random is 2/5. If there are 12 blue pens, how many red pens are in the box?
Answer: 8
With r red pens, r/(r + 12) = 2/5 gives 5r = 2r + 24, so r = 8.
Common wrong answers
- If you answered 20
- 20 is the total number of pens.
- If you answered 30
- 30 treats 12 as 2/5 of the total.
- If you answered 4.8
- 4.8 takes 2/5 of 12.
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