Think of a Number/Examples/Rhind Papyrus 28

Example of Think of a Number puzzles
Problem $28$ of the is as follows:
 * $\dfrac 2 3$ is to be added.


 * $\dfrac 1 3$ is to be subtracted.


 * There remains $10$.

Solution
This can more clearly be expressed as:


 * I think of a number.


 * I add to it $\dfrac 2 3$ of the number.


 * I then subtract $\dfrac 1 3$ of the sum.


 * My answer is $10$.


 * What number did I think of?

The number was $9$.

Proof
Let $x$ be the number first thought of.

We have: