A box has 20 paper slips having a number from 1-20 written on it. One slip is chosen at random from the box. The probability that the chosen slip has a composite number on it, is: 

$\frac{11}{20}$

Numbers on slips

$1,2,3,\dots,20$

Composite numbers between $1$ and $20$ are

$4,6,8,9,10,12,14,15,16,18,20$

Total composite numbers $=11$

Total possible outcomes $=20$

Probability

$P(\text{composite})=\frac{11}{20}$

The probability is $\frac{11}{20}$.