Idon't really get what it mean by "a group of 10 friends had lunch together at a restaurant. the meal cost a total of $99.50, including a 15% tip. how much was the total bill for lunch before tip? "

Don't mind how many frieds there were, they question doesn't ask about that

so
original=100%
plus a 15% tip
100%+15%=115%

the total WITH tip was 99.50

therefor
99.50=115% of total befoer tip

percent means parts out of 100
115%=115/100=1.15

'of' means multipy

99.50=1.15 times totalbeforetip
divide both sides by 1.15
86.521=totalbeforetip
round
86.52=totalbefretip

totalbeforetip=$86.52