Question

Mark the correct alternative in the following question:

If the first three terms of a proportion are 3, 5 and 21, respectively, then its fourth term is

(a) 21 (b) 35 (c) 15 (d) None of these

Answer 1

$$\begin{gathered} \mathrm{Let}\mathrm{the}\mathrm{fourth}\mathrm{term}\mathrm{be}x. \\ \mathrm{As},3:5::21:x \\ \Rightarrow \frac{3}{5}=\frac{21}{x} \\ \Rightarrow 3x=21\times 5 \\ \Rightarrow x=\frac{21\times 5}{3} \\ \Rightarrow x=7\times 5 \\ \therefore x=35 \end{gathered}$$

So, the fourth term is 35.

Hence, the correct alternative is option (b).