When Cheenu was a boy he could run 15 miles in 3 hours and 30 minutes. As an old man he can now walk 10 miles in 4 hours. How many minutes longer does it take for him to walk a mile now compared to when he was a boy?<spanclass=′latex−bold′>(A)</span>6<spanclass=′latex−bold′>(B)</span>10<spanclass=′latex−bold′>(C)</span>15<spanclass=′latex−bold′>(D)</span>18<spanclass=′latex−bold′>(E)</span>30