If 7, 12, 21 and x are in proportion, the value of x is:

Correct option is A

Given:

7, 12, 21 and x are in proportion

Solution:

When four number $$a,b,c$$ and $$d$$ are in proportion they the equation is:

$$\frac{a}{b}=\frac{c}{d}$$

Here $$a=7,b=12,c=21$$ and $$d=x$$, Substituting these value in the proportion equation:

​$$\frac{7}{12}=\frac{21}{x}$$

Cross multiply to solve for $$x$$ 

​$$7x= 21\times12$$

$$7x=252$$

$$x=\frac{252}{7}=36$$

$$x=36$$

Thus the correct answer is (A)