Ratio 3 : 5 now, 3 : 4 later — Aptitude Question
Ratio 3 : 5 now, 3 : 4 later: The ages of A and B are in the ratio 3 : 5. After 9 years the ratio becomes 3 : 4. Answer and worked solution on CodeKairo.
- Topic: Problems on Ages
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 75 seconds
Question
The ages of A and B are in the ratio 3 : 5. After 9 years the ratio becomes 3 : 4. What is B's present age?
Options
- 9
- 12
- 15
- 20
Show the answer and the worked solution
Correct answer: C. 15
Worked solution
- (3x + 9)/(5x + 9) = 3/4 → 12x + 36 = 15x + 27 → 3x = 9 → x = 3.
- B = 5 × 3 = 15.
- Check: 18/24 = 3/4. ✓
Answer: 15.
The idea
Same shape as before. Substituting x back into the right person's expression is the only place to slip.
Shortcut. Scale 3 : 4 up to 6 : 8 so both ratios show a gap of two parts. A then goes from 3 parts to 6 in nine years, so one part = 3 and B = 5 × 3 = 15.
Common trap. 9 is A's present age (3 × 3), from putting x = 3 into A's term instead of B's; it also echoes the nine years in the question.