There are 5 hats. Three blue, two red. They are placed in a box and mixed up. Three people are blind folded and standing in a line one behind the other facing forward. Each person reaches into the box, one at a time and withdraws a hat which is then placed on their head. Their blind folds are then removed. The last of the persons in line (who sees the other two) says "I don't know the colour of my hat". After this the second says "I don't know the colour of my hat". The first person in the line then works out the colour of their hat, how?