There is a popular myth that after drinking a bottle of water, the last bit is mostly backwash. Well I decided to try and test it, but got stumped. Lets call y the amount of backwash in the bottle Lets call x the number of sips taken The volume of the bottle will be 1000mL Assume each sip is 20mL Assume that each sip backwashes 0.05mL into the bottle dy/dx= 0.05 -20( y / (1000 -(20-0.05) x ) ) I can't separate variables here, so I do not know what to do. This is not a homework problem, I was just wondering if anyone could help me solve this differential equation. At 51 sips there will be nothing left in the bottle.