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

A. 3

B. 5

C. 7

D. 9

Solution:

Let the number be x
Then,

\begin{aligned} \Leftrightarrow x - 4 = \frac{21}{x} \\ \Leftrightarrow x^{2} - 4 x - 21 = 0 \\ \Leftrightarrow (x - 7) (x + 3) = 0 \\ \Leftrightarrow x = 7 \end{aligned}

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