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.(adsbygoogle = window.adsbygoogle || []).push({});

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.

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