Last week I spent half of my money on a new jacket and then I spent half of that amount on some new trousers.
I was left with £32.94. How much did I spend?
£98.82: I started with £131.76 and I spent £65.88 on a new jacket, and then £32.94 on the trousers, a total spend of £98.82.
At the end I was left with £32.94, which was half of my remaining money, so before buying the trousers I must have had £65.88. Again, this was half of my money, so before buying the jacket I must have had £131.76 at the start. As I now had £32.94 remaining, I must have spent £98.82. QED.