Some additional remarks on exercise 10. 1. b) The problem was solved by first drawing a line through c3 and c and finding its intersection with the line from c1 to c2. From this p3 could be found. (Then using the ratio R12=p1/p2 the problem was solvable.) There was some discussion then if one could have started from some other point, say c1 and draw a line through c and thus finding p1. Then one would do the same for c2 and find p2. This is indeed possible as well, which can be intuitively understood... The percentage p3 is however _not_ proportional to the line from c3 to c, but instead to the line c to c0. Because the larger c3-c is, the further away the point c is from c3, an the smaller the percentage p3. (This fact can also be seen from exercise 1.a), where the percentage of p1 is proportional to the distance c-c2, not c-c1.) In the exercise session I misleadingly highlighted the c3-c line when talking about the percentage p3. Looking at the image in the model answer, the line c-c0 is shorter, about 1/3, or 33%, which seems more plausible. Proving that p1, p2 and p3 calculated in this way indeed sum up to 100% in the general case is however a bit complicated :) 3. The CMY colour model use the colours cyan magenta and yellow as it base colours. These are often called the secondary colors of light (since they are at the opposite ends of the RGB colour cube), but also the primary colours of pigments. Cyan for example is the opposite of red. This means that if we shine a white light (which is composed of equal amounts of red, green and blue) on a cyan surface it will absorb only the red part of that light and reflect the others. Similary magenta is the colour that absorbs green and reflect others, yellow absorbs blue. This is why these colours are more natural to use when for example printing on a white paper area. The CMYK colour space is often used in real printing applications. The K stands for "key", but is in practice the pure black colour as discussed in the exercise session. See e.g. http://en.wikipedia.org/wiki/CMYK