The two tins are different shapes (though the both will hold exactly a gallon) and they are not symmetric shapes, so it is not possible to tell if they are, say, half full by tilting them at an angle. The painter cannot make marks on the tins either .
He needs all ten pints to paint the room, unfortunately, he doesn't have a mixing vessel large enough to hold all the paint - the only other container he has is a pint glass.
By pouring paint back and forth, mix the paint correctly using just these containers.
