Chapter 11 - Probability and Statistics - 11-2 Probability - Practice and Problem-Solving Exercises - Page 685: 16

$\frac{4}{5}$

$$P(\text{red or blue}) = P(\text{red}) + P(\text{blue}) - P(\text{red and blue})$$ Because only one marble is being drawn $$P(\text{red and blue}) = 0$$ We obtain: $$P(\text{red or blue}) = P(\text{red}) + P(\text{blue})= \frac{30}{30+50+20} + \frac{50}{30+50+20} = \frac{80}{100} = \frac{4}{5}$$