Assume that 2 empty cans, a 30l-can and a 50l-can, are kept on standby, but no scale is on hand.
1. Fill 50l-can with water to capacity, and then remove the water in 50l-can into 30l-can to capacity: 30l-can is full and 50l-can holds 20 litres of water inside.
2. Empty 30l-can, and then remove 20 litres of water left in 50l-can into 30l-can: 30l-can holds 20 litres of water inside and 50l-can is empty.
3. Fill 50l-can with water to capacity again, and then remove the water in 50l-can into 30l-can to capacity: 10 litres of water in 50l-can is removed into 30l-can.
Now we have 40 litres of water in 50l-can and 30 litres of water in 30l-can.