Question 255171
First find the area of each ring or section. You can so this in each case by finding the area of the circle whose circumference is the outer part of the ring and substract the area of the circle whose circumference is the inner part of the ring. The formula for the area of a circle is A = pi*r^2 where r (the radius) is 1/2 of the diameter.

Once you have the area of each ring (or section) the probability of a dart landing in that ring or section is given by the area of the ring or section divided by the area of the entire dart board (that is, the percent the ring or section represents of the total area of the dart board).

Note that we have assumed that a dart would always hit somewhere on the dart board and also that each individual "point" on the board was equally likely to be hit by random throw :)