Volume · Superchallenge problem

A fish tank is $60$ cm long, $30$ cm wide and $40$ cm tall. A thin wall $20$ cm tall stands across the tank $20$ cm from the left end, so it splits the floor into a left part $20$ cm long and a right part $40$ cm long. Water is poured into the left part at $3$ liters per minute. How many minutes after pouring starts is the water in the right part $10$ cm deep? ($1$ liter $= 1000$ cubic cm.)