Answer
To find the probability of the event E we use
$p(E)=\frac{\text { Number of outcomes by } E}{\text { Total number of outcomes }}$
then
a-) $p(100\% free)=\frac{1}{\text { 50 }}=0.02$
b-) $p(0\% free)=\frac{10}{\text { 50 }}=0.2$
c-) $p(E\geq 20\% free)=\frac{20}{\text { 50 }}=0.2$
Work Step by Step
c-) $p(E\geq 20\% free)=p(100\% free)+p(50\% free)+p(40\% free) +p(30\% free)+p(20\% free)$
$$=\frac{1}{\text { 50 }}+\frac{1}{\text { 50 }}+\frac{3}{\text { 50 }}+\frac{5}{\text { 50 }}+\frac{10}{\text { 50 }}=0.2$$
