There are three buckets: one red, one blue and one yellow. They each hold a maximum of $5$ litres.
Liquid is measured carefully in whole litres and poured into the buckets, a different number of litres in each one.
If the liquid in the red bucket was poured into the blue bucket, it would then contain the same amount of liquid as the yellow bucket.
Half the contents of the yellow bucket is the same as twice that in the red bucket.
How much liquid is there in each bucket?
You may now want to try More and More Buckets
This problem is also available in French: Question de chaudières.