Just think, so Roller charges for 2.75 PLUS 15 dollar per person; that means, the values will add. Maria has 96 dollars, that is the answer.
Then write the equation like this: 2.75+15*x (x means a number of people) = 96
You add 2.75+15 which are 17.75 and then divide 96/17.75 to find x (or amount of people) and it gives you 5.408, you can round it to 5 people. I hope you understand :)