[tex]x=primul\ raft\\
\frac{x}{4}=al\ doilea\ raft\\
\frac{1}{3}(x+\frac{x}{4})=al\ treilea\ raft\\
x+\frac{x}{4}+\frac{1}{3}(x+\frac{x}{4})=80\\
\frac{5x}{4}+\frac{x}{3}+\frac{x}{12}=80\\
15x+4x+x=960\\
20x=960=\ \textgreater \ x=48\ carti\ pe\ primul\ raft\\
\frac{48}{4}=12\ carti\ pe\ al\ doilea\ raft\\
\frac{1}[3}(48+12)=\frac{1}{3}*60=20\ carti\ pe\ al\ doilea\ raft.[/tex]