Twenty one times of a positive number is less than its square

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

1. 25
2. 26
3. 42
4. 41

Correct Option: A

Suppose, the positive number be x.
According to the question,
x² – 21x = 100
⇒ x² – 21x – 100 = 0
⇒ x² – 25x + 4x – 100 = 0
⇒ x (x – 25) + 4 (x – 25) = 0
⇒ (x – 25) (x + 4) = 0
⇒ x = 25 because x ≠ –4