x-lungimea drumului
I zi parcurge:x-20x/100=x-x/5=(5x-x)/5=4x/5
II zi:4x/5-30/100·4x/5=simplif=4x/5-6x/25=(20x-6x)/25=14x/25
III zi:14x/25=560
x=560·25/14
x=1000 km lungimea drumului
verificare
I zi:1000-20/100·1000=1000-200=800
II zi:800-30/100·800=800-240=560
III zi:560