Civil Service Exam Reviewer · Ratio and Proportion · hard
A box has red and blue balls in the ratio 7 : 3. After 2 red balls are removed, the ratio becomes 47 : 21. How many blue balls are in the box?
- 49
- 21
- 70
- 23
Answer
B. 21
Solution
Red = 7k, blue = 3k. (7k − 2)/3k = 47/21 → k = 7. Blue = 3 × 7 = 21.
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