Question 33
There are only coins of $11$ pesos and $13$ pesos in a country (and no bills). An ice-cream shop is about to open and there is a line of customers waiting. Every customer wants to buy a cone of ice-cream and has exactly $155$ pesos. The cone costs $12$ pesos. The salesperson wants to attend everyone by giving back the exact change without borrowing or exchanging money. Find the minimum amount of money she needs to have in advance in order to do so.