Assume you are going to give a presentation to about 30 people in a medium sized room. In order to maximize the contrast, you used black letters on white background in your slides. You have also learned that the projector in the room will show the white background with a brightness of 90 Ft-L and the black letters with a brightness of 10 Ft-L. Considering the room size and your audience, you decide to make the font size large enough so that a person with 20/40 vision will be able to read the letters displayed on the screen with your slides with 95% accuracy from 20 m away. What is the minimum width of the displayed letter strokes in centimetre?