x⇒ litri benzina
x·25%=x· 25/100=x/4 ⇒a consumat in cursa I
x-x/4·20%=3x/4·1/5=3x/20 a consumat in cursa II
(x/4+3x/20) = a consumat in ambele curse
x-(x/4+3x/20)=x/4+3x/20+6
x-x/4-3x/20=x/4+3x/20+6
20x-5x-3x=5x+3x+120
x=120/4
x=30 l benzina au fost la inceput in rezervor