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

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Explanation

  • A positive number when decreased by 4.
  • Is equal to 21 times the reciprocal of the number.
  • The number will be figure out in this way.

Let suppose the positive number is y.

21 times the reciprocal of the number

21 x 1/y

Positive number when decreased by 4

y – 4 = 21 x 1/y ________ (i)

Through equation (i) we can easily find out the value of y (y = 7).

To Find

Number = ?

Solution

Let suppose

Number = y

According to the given condition

y – 4 = 21 x 1/y

y(y – 4) = 21

y2 – 4y = 21

y2 – 4y – 21 = 0

y2 – 7y + 3y – 21 = 0

y(y – 7) + 3(y – 7) = 0

(y – 7)(y + 3) = 0

y + 3 = 0             &                y – 7 = 0

y = -3 (not possible)             &                y = 7 (possible)

Required Number = 7 answer

Conclusion

So, the required number is 7 which when decreased by 4 is equal to 21 times the reciprocal of the number.

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