You need to know the relative sizes of the double and treble portions of the dartboard segments. It is sufficient to know what the proportions are for just one twentieth segment. The chances also assume that the dart player is unskilled for the chances to be random. A skilled player will not necessarily be aiming for double or treble unless he/she is going for 180 using three darts (3*treble-20). One segment has an area of (pi)r^2/20. If the internal radii for the double and treble zones are d and t respectively then the size of the area for hitting a double divided by the segment area is (r^2-d^2)/r^2. For the treble it's (d^2-t^2)/r^2. If r=30cm, d=29cm and t=28cm, then the chances of hitting a double are 59/900=0.06556 (6.56%), or a treble=57/900=0.06333 (6.33%).