Mang Ruben has only an 11-liter can and a 5- liter can. How can he measure out exactly 7 liters of water?
Fill 11-litre can twice from 5-litre can. Now add 1 litre from 5-litre can, 4 litres left. Empty 11-litre can, Now fill it up with the 4 litres remaining in the 5 litre can.
Add 5 litres from 5-litre can to 11-litre can giving 9 litres in it. Now add 2 litres from 5-litre can, 3 litres left. Empty 11-litre can, Now fill it up with the 3 litres remaining in the 5 litre can.
Add 5 litres from 5-litre can to 11-litre can giving 8 litres in it. Now add 3 litres from 5-litre can, 2 litres left. Empty 11-litre can, Now fill it up with the 2 litres remaining in the 5 litre can.
Add 5 litres from 5-litre can to 11-litre can giving 7 litres in it.
Job done!