Two intersecting circles in a rectangle, one blue, one yellow with green the colour for the intersection of the two circles. 12 is the number of 'DVDs only' in the blue region, 6 is the number of 'MP3s only' in the yellow region and in the green region 36 is the number of 'both DVD and MP3 players'. The number with neither (outside the circles in the rectangle) is 36.