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.**

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**