A positive number when decreased by 4 is equal to 21 times the reciprocal of the number. The number is ?

Answer: Option C

Let the number be x
Then,
$$\begin{array}{rl}& \iff x-4=\frac{21}{x}\\ & \iff x^2-4x-21=0\\ & \iff \left(x-7\right)\left(x+3\right)=0\\ & \iff x=7\end{array}$$