CA Foundation · Quantitative Aptitude · Ratio and Proportion, Indices and Logarithms
The fourth proportional to 12, 18 and 30 is:
The fourth proportional is 45. For a : b = c : x, x equals b times c divided by a. Here x = 18 × 30 / 12 = 540 / 12 = 45. Inverting the ratio would wrongly give 20.
- A45Correct
- B36
- C20
- D50
Explanation
The fourth proportional x satisfies 12 : 18 = 30 : x. So x = 18 × 30 / 12 = 540/12 = 45. The option 20 results from inverting the ratio (12 × 30 / 18), which is the wrong cross-multiplication.
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