Fill 2L can with water and empty into 5L can.
Repeat.
There will be 4L of water in the 5L can.
Starting with a 3L and a 5L can, how to get 4L of water into the 5L can, assuming an unlimited supply of water.
- Fill 5L can with water.
- From 5L can, fill 3L can from 5L can, leaving 2L in the 5L can.
- Discard contents of 3L can.
- Empty the 2L of water from the 5L can into the 3L can.
- Fill the 5L can.
- Top up the 3L can from the 5L can, leaving 4L in the 5L can.