When a positive number is decreased by 4, it is equal to 21 times the reciprocal of the number. Find the number.

Correct option is C

Solution:

Let the number be x .
​$$x - 4 = 21 \cdot \frac{1}{x}\\$$​

​$$x - 4 = \frac{21}{x}$$​​
​$$x^2 - 4x = 21\\x^2 - 4x - 21 = 0\\(x - 7)(x + 3) = 0\\x = 7 or x = -3\\$$​​
Since x is positive, the solution x = 7
Thus, the number is 7.