notam x cantitatea initiala
x-[tex] \frac{x}{2} [/tex]-3=40%·x
x-[tex] \frac{x}{2} [/tex]-3=[tex] \frac{40}{100} [/tex]·x
x-[tex] \frac{x}{2} [/tex]-3=[tex] \frac{2}{5} [/tex]·x aducem la acelasi numitor comun 10
10·x-5·x-30=2·2·x
5·x-30=4·x
5·x-4·x=30
x=30
deci volumul initial era de 30l