There are $8$ prime numbers in total.  So, answer $= \frac{8}{20} = \frac{2}{5}$ 
There are $20$ cards numbered from $1$ to $20$, respectively. What is the probability of taking out a card with a prime number? 
a. $\dfrac{3}{10}$ 
b. $\dfrac{7}{20}$ 
c. $\dfrac{2}{5}$ 
d. $\dfrac{9}{20}$ 