If you add up all the band members, you'll get a sum of 40. If you divide this figure into 360 degrees (the angle measure of a circle), you'll get a quotient of 9. Therefore, if you multiply the amount of members using each instrument by 9 you will get the angle measure that each instrument will take on the circle graph.
French Horn = 6 * 9 = 54 degrees
Trumpet = 15 * 9 = 135 degrees
Tube = 8 * 9 = 72 degrees
Trombone = 11 * 9 = 99 degrees