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

Expert Answers

An illustration of the letter 'A' in a speech bubbles

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.

Approved by eNotes Editorial Team