A painter needed 6 pints of grey paint, so he mixed 4 pints of black paint with 2 pints of white paint. This turned out to be too dark a grey and he realised that what he really needs is a mixture that has 4 pints of white with 2 of black.

The painter understands that he will have to throw away some of the paint, but wants to get the correct amount of the right mixture with as little waste as possible. How should he do it?