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The ratio of the third proportional to 12 and 24 to the mean proportional between 9 and 36 is:

  • A.3 : 5
  • B.7 : 5
  • C.8 : 3
  • D.9 : 4

Answer: C

The ratio is 8 : 3. The third proportional to 12 and 24 is the number c with 12 : 24 = 24 : c, and the mean proportional between 9 and 36 is the square root of their product.

\[ c = \frac{24 \times 24}{12} \]

\[ c = 48 \]

\[ \text{mean proportional} = \sqrt{9 \times 36} \]

\[ \sqrt{324} = 18 \]

\[ 48 : 18 = 8 : 3 \]

The options 3 : 5, 7 : 5 and 9 : 4 come from mixing up the two proportionals, for example using the mean of 12 and 24 or the third proportional of 9 and 36, and none of them simplifies from 48 : 18.

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