There are $20$ cards numbered from $1$ to $20$, respectively. What is the probability of taking out a card with a prime number? 
Overseas Competition->Knowledge Point->Counting Modules->Statistics and Probability->Questions Involving Probability->Basic Concepts of Probability
There are $8$ prime numbers in total.  So, answer $= \frac{8}{20} = \frac{2}{5}$ 