1. Problem 19: Probability of a purple marble or a blue marble (a) Formula: $p(p) + p(b)$ (b) Substitute values: $\frac{1}{12} + \frac{1}{5}$ (c) Find common denominator and add: $$\frac{1}{12} + \frac{1}{5} = \frac{5}{60} + \frac{12}{60} = \frac{17}{60}$$ 2. Problem 18: Probability of a yellow marble and then a blue marble if the yellow marble is replaced (a) Formula: $p(y) \times p(b)$ (b) Substitute values: $1 \times \frac{1}{5}$ (c) Multiply: $$1 \times \frac{1}{5} = \frac{1}{5}$$ 3. Problem 20: Probability of a purple marble and then a blue marble if the purple marble is replaced (a) Formula: $p(p) \times p(b)$ (b) Substitute values: $\frac{1}{12} \times \frac{4}{5}$ (c) Multiply: $$\frac{1}{12} \times \frac{4}{5} = \frac{4}{60} = \frac{1}{15}$$