Question

The first three terms of a proportion are 12, 21 and 8 respectively. Then 4^th term is

(a) 18
(b) 16
(c) 14
(d) 20

Answer 1

Let the 4^th term be x.

According to the question,
$$\begin{gathered} 12:21::8:x \\ \Rightarrow \mathrm{Product}\mathrm{of}\mathrm{extreme}\mathrm{terms}=\mathrm{product}\mathrm{of}\mathrm{mean}\mathrm{terms} \\ \Rightarrow 12x=8\times 21 \\ \Rightarrow 12x=168 \\ \Rightarrow \frac{12x}{12}=\frac{168}{12} \\ \Rightarrow x=14 \end{gathered}$$

Hence, the correct option is (c).