Probably the 36 for 4$, but it also depends on the fluid OZ's of the bottle.
You do 9x27=243 then take 2x31=62 add 243+62=305 then take 305 and subtract it from 28 305-28=277
so if i did it right you should get n=277
19+13=32
32+63=95
95-4=91
91+10=101
This means that 101 people are now on the bus.
HOPE THIS HELPS <3