`5/7` of a number is 15. What is the number?

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Define the number by the variable x.

Therefore 5/7 of "what number" is 15 means `5/7* x = 15`  

In order to isolate the variable x, we can multiply both sides by the reciprocal of `5/7`  which is `7/5.`

A number multiplied by its reciprocal is always equal to 1.

`7/5* 5/7 x = 15* 7/5`

`x = 21`

`5/7` of 21 is 15.

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Let the number 5/7 of which is 15 be X.

(5/7)*X = 15

=> X = 15*(7/5)

=> X = 21

5/7 of 21 is equal to 15

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