4. Elena bikes 20 minutes each day for exercise. Write an equation to describe the relationship between her distance in miles. D. and her biking speed, in miles per hour, when she bikes: a. at a constant speed of 13 miles per hour for the entire 20 minutes 143 b. at a constant speed of 15 miles per hour for the first 5 minutes, then at 12 miles per hour for the last 15 minutes