AlgebraMediumBlue marbles from a conditional relation
Algebra practice question
A bag holds 24 marbles, each red or blue. If there were 3 more red marbles, there would be twice as many red marbles as blue ones. How many blue marbles are in the bag?
Answer: 9
With r + b = 24 and r + 3 = 2b, substituting r = 24 − b gives 27 − b = 2b, so b = 9.
Common wrong answers
- If you answered 8
- 8 uses r = 2b without the extra 3.
- If you answered 12
- 12 halves the bag.
- If you answered 15
- 15 is the number of red marbles.
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