A positive number when decreased by 4, is equal to 21 times

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

Let the number be N.
According to the question,

N − 4 =	21
	N

⇒ N² – 4N = 21
⇒ N² – 4N – 21 = 0
⇒ N² – 7N + 3N – 21 = 0
⇒ N (N – 7) + 3 (N – 7) = 0
⇒ (N + 3) (N – 7) = 0
⇒ N = 7 because N ≠ – 3.