its quite simple. for every white bucket of paint he uses, he uses 3 buckets of black paint as well. so if he made 24 litres of grey paint then he would have had to use 24 buckets of white paint. but, this might be a trick question because its saying he made 24 "litres" but the unit it started with was "buckets" so it might be trying to trick you because we dont know how much paint is in one "bucket"
