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1. The fourth proportional to 5, 8, 15 is:

A. 18

B. 24

C. 19

D. 20

Solution:

\begin{aligned} Let the fourth proportional to 5, 8, 15 be x \\ Then, 5 : 8 : 15 : x \\ \Rightarrow 5 x = 8 \times 15 \\ x = \frac{8 \times 15}{5} = 24 \end{aligned}

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