If 9.6 : 16.8 :: 18 : x, what is the value of x ?

Correct option is D

Given: 

​9.6 : 16.8 :: 18 : x

Solution:

​We are given the proportion:

​$$\frac{9.6}{16.8} = \frac{18}{x}$$ 

by cross-multiplying:

$$9.6 \times x = 16.8 \times 18$$ 

$$x = \frac{16.8\times{18}}{9.6}$$ 

​$$x = 31.5$$ 

​Thus, the value of $x$ is 31.5 .​