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