he has 50 butts so he can make 16 cigarettes with it (he's left with 2 butts); then he smokes those 16 cigarettes and has 16 butts (and 2 that were left last time); from those 18 cigarettes he can make 6 cigarettes; then he's left with 6 butts, which from he can make another 2 cigarettes and only then he's left with 2 butts that wait for another third.
so 16 + 6 + 2 = 24
right?