Question 363562
You need to find the area of the triple 20 region {{{A[t20]}}}.
Then the probability of hitting a triple 20 is the total area of a triple 20 region divided by the area of the entire dartboard {{{A[t]}}}.
{{{A[t]=pi*(D^2/4)=pi*(18)^2/4=(81/4)pi}}}
{{{P=A[t20]/((81/4)pi)}}}
{{{highlight(P=(4*A[t20])/(81*pi))}}}