Brain teasers. if I start with £50

If I start with £50

If I spend £20 ......... I have £30 left
If I spend £15.......... I have £15 left
If I spend £9.............. I have £6 left
If I spend £6 ............. I have £0 left

Adds up to £50 ........ add up too £51

Any ideas?


a

If your ending balance is zero what you spent has to equal what you started with. Why does the total of what you have left after each transaction necessarily have to be the same?

Start with £50

If I spend £50 ......... I have £0 left

Adds up to £50 ........ add up to £0

So?