AlgebraEasy–MediumJar of marbles with a fractional part
Algebra practice question
A jar holds x marbles, and two-thirds of them are red. After 4 red marbles are taken out, 6 red marbles remain in the jar. How many marbles were in the jar at the start?
Answer: 15
Red marbles: (2/3)x. After removing 4: (2/3)x − 4 = 6, so (2/3)x = 10 and x = 15.
Common wrong answers
- If you answered 3
- 3 comes from (2/3)x = 2, subtracting 4 instead of adding it back.
- If you answered 10
- 10 is the number of red marbles at the start, not the whole jar.
- If you answered 6.67
- 6.67 multiplies 10 by 2/3 instead of dividing by it.
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