Problem #$3$: In a mixing vat these are initially $100$ litres of volatile liequid. $4$ litres of fresh liequid are added per bour, bur $20\%$ of the curent consens of the vat is lost each hour due to etaporation. How many litres of liequd ase there after $4$ bours?

Aroblem $=3$ :

Ener yaut artawet sumbelicaliv,

is $n$ these cxampless