Problem-Solving and Data AnalysisEasy–MediumCount from a probability
Problem-Solving and Data Analysis practice question
A bag contains 30 marbles, each either red or blue. The probability of drawing a red marble at random is 2/5. How many blue marbles are in the bag?
Answer: 18
Red marbles: (2/5)(30) = 12. Blue marbles: 30 − 12 = 18.
Common wrong answers
- If you answered 6
- 6 is 30/5, one 'fifth' of the bag; red is two fifths (12) and blue is three fifths (18).
- If you answered 12
- 12 is the number of RED marbles. The question asks for blue.
- If you answered 20
- 20 is 30 − 10 or 2/3 of 30; the red fraction is 2/5, so blue is 3/5 of 30 = 18.
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