Part A: divide the miles she biked in the amount of time she took. so, 21/4= 5.25. Her average speed is 5.25 miles per hour.
Part B: If she bikes at the same rate (21 miles in 4 hours), then she would take 8 hours to bike 42 miles because 21+21=42 and since she takes 4 hours to bike 21 miles, then it would take her another 4 hours to bike another 21 miles