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

Let the number be x,
Then x - 4 = 21/x
⇒ x² - 4x - 21 = 0
⇒ x² - 7x + 3x - 21 = 0
⇒ x(x - 7) + 3(x - 7) = 0
⇒ (x - 7) (x + 3) = 0
∴ x = 7 (Neglecting = -3)