Question

Mark (✓) against the correct answer
The third proportional to 9 and 12 is

(a) 10.5
(b) 8
(c) 16
(d) 21

Answer 1

(c) 16

Suppose that the third proportional is x.
Then, 9 : 12 :: 12 : x

$$\begin{gathered} \Rightarrow 9\times x=12\times 12(\mathrm{Product}\mathrm{of}\mathrm{extremes}=\mathrm{Product}\mathrm{of}\mathrm{means}) \\ \Rightarrow 9x=144 \\ \Rightarrow x=16 \end{gathered}$$