Find the fourth proportion to 3, 7 and 9.
Correct answer: C. 21
- A. 63
- B. 31
- C. 21
- D. 27
Explanation
The fourth proportion x satisfies 3:7 = 9:x. Cross-multiplication gives 3x = 7 x 9 = 63, so x = 21. The likely mistake is choosing 63 by using the cross-product as the answer instead of dividing by 3.
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About Ratio and Proportion
Ratios compare quantities in the same units, while proportions state that two ratios are equal. Problems cover simplification, division in a given ratio, direct and inverse variation, scale, and missing terms, with attention to the difference between a ratio and a fraction representing part of a whole.
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