Think of a number.

Double it.

Add 10..

Halve it.

Take away you original number.

Is your answer 5?

Of course it is!

To see why this works and why it does not matter what number you thought of, left the number you thought of be called

\[x\]

.When you double it you have

\[2x\]

When you add 10 you have

\[2x+10\]

Halving gives

\[x+5\]

Subtract you original number

\[x\]

gives 5.