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

A. 3

B. 5

C. 7

D. 9

Answer: Option C

Solution(By Examveda Team)

Let the number be x
Then,
$$\eqalign{ & \Leftrightarrow x - 4 = \frac{{21}}{x} \cr & \Leftrightarrow {x^2} - 4x - 21 = 0 \cr & \Leftrightarrow \left( {x - 7} \right)\left( {x + 3} \right) = 0 \cr & \Leftrightarrow x = 7 \cr} $$