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