Twenty one times of a positive number is less than its square by 100. The value of the positive number is:

Answer: Option A

Let the number be = x
Then according to question
21 × x + 100 = x²
x² - 21x - 100 = 0
x² - 25x + 4x - 100 = 0
x(x - 25) + 4(x - 25) = 0
(x - 25)(x + 4) = 0
x = -4 is not possible
x = 25