The change in Kai and their friends' amount of money after paying the bill is represented by -51 euros, indicating a decrease in their money.
When Kai and their friends pay the bill of 51 euros, their collective amount of money decreases by that amount. The correct number to represent this change in their money is -51 euros. A positive number would indicate an increase in money, whereas a negative number indicates a reduction. So, when they hand over cash or make a payment, they are giving away money, hence the amount they possess decreases.