You are in the possession of two bottles, one with a capacity of 7 litres and one with a capacity of 11 litres. Next to you is an infinitely large tub of water.
You need to measure exactly 2 litres in one of the bottles.
You are only allowed to entirely empty or fill the bottles. You can't fill them partially since there is no indication on the bottles saying how much liquid is in them.
How do you measure exactly 2 litres?
Fill the 7 litre bottle completely and pour the whole thing to the 11 litre bottle.
Fill the 7 litre bottle completety again and pour the water to the 11 litre bottle until it becomes full.
Now the 7 litre bottle contains 3 litres.
Empty the 11 litre bottle and then pour the 3 litres into the 11 litre bottle.
Fill the 7 litre bottle completely and pour the whole thing to the 11 litre bottle.
Fill the 7 litre bottle completely again and pour the water to the 11 litre bottle until it becomes full.
Now the 7 litre bottle contains 6 litres.
Empty the 11 litre bottle and then pour the 6 litres into the 11 litre bottle.
Fill the 7 litre bottle completely and pour the water to the 11 litre bottle until it becomes full.
Now, the 7 litre bottle contains 2 litres.
An other possible solution would be to start by filling the 11 litre bottle as shown below.
