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.

The weight of a radioactive isotope was 96 grams at the start of an experiment. after one hour, the weight of the isotope was half of its initial weight. after two hours, the weight of the isotope was half of its weight the previous hour. if this pattern continues, which of the following graphs represents the weight of the radioactive isotope over time? a. graph w b. graph x c. graph y d. graph z