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

None of these

Given equation:

x - 4 = 21(1/x)

Multiply both sides by x:

x² - 4x = 21

Rearrange the equation:

x² - 4x - 21 = 0

Factor the quadratic equation:

(x - 7)(x + 3) = 0

Solve for x:

x = 7 (since x is a positive number)