20. I roll two standard six-sided fair dice. At least one of the scores obtained on the dice is 3 . What is the probability that both of the scores on the dice are 3 ?
A $\frac{1}{12}$
B $\frac{1}{11}$
C $\frac{1}{6}$
D $\frac{1}{3}$
E $\frac{1}{4}$
20. B There are 36 different equally likely possible outcomes $(a, b)$ for the two dice. The number of these which involve only $1,2,4,5$ or 6 is $5 \times 5=25$. So there are just eleven involving a 3 , one of which is $(3,3)$. So the probability that both are 3 , given that at least one is 3 , is $\frac{1}{11}$.