Prime number

Jan wrote any number from 1 to 20. What is the probability that he wrote the prime number?

Correct result:

p =  0.4

Solution:

$$\begin{gathered} p:2;3;5;7;11;13;17;19 \\ p={\displaystyle \frac{8}{20}}={\displaystyle \frac{2}{5}}=0.4 \end{gathered}$$

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