The trick is to have bill and 15% gratuity for service. so if x is your bill. The group is paying 1.15x as their Bill.
On the GMAT try to use fractions they will help with the calucations.
Bill for 15 => 207/(115/100)
Bill for 1 => 207*100/ ( 115*15)
The fraction looks crazy to solve, but on the GMAT they wont test your for calculator skills generally. So you can plug in values or solve them. Both would take same time.
Most importantly, dont do it in your. Dont Tax your Brain. You need it for the verbal section