A prime number has exactly $2$ factors: itself and $1$ The numbers from $1$ to $20$ inclusive are written on $20$ cards. Write down the proportion of all the cards that have a prime nuniber written on them, giving your answer as a decimal. 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples->Basic Concepts of Factors and Multiples Overseas Competition->Knowledge Point->Number Theory Modules->Prime Factorization->Prime Factorization (Equations)