Fourth proportional to 5, 8 and 15 — Aptitude Question
Fourth proportional to 5, 8 and 15: What is the fourth proportional to 5, 8 and 15? Answer and worked solution on CodeKairo.
- Topic: Ratio & Proportion
- Section: Quantitative Aptitude
- Difficulty: Easy
- Time target: 40 seconds
Question
What is the fourth proportional to 5, 8 and 15?
Options
- 18
- 20
- 24
- 30
Show the answer and the worked solution
Correct answer: C. 24
Worked solution
- 5 : 8 = 15 : x → 5x = 8 × 15 = 120.
- x = 24.
Answer: 24.
The idea
Fourth proportional to a, b, c is bc/a. Keep the pairing right: the first two form one ratio and the last two the other.
Shortcut. 15 is 3 × 5, so the fourth term is 3 × 8 = 24.
Common trap. "18" — adding the difference 8 − 5 = 3 to 15; a proportion works by multiplying, not adding.