David cycles 12 miles to work. He leaves home at the same time each day, cycles at a steady speed of 8 miles per hour, and arrives just as work starts. Today he stopped for coffee and cycled the first 3 miles at a speed of only 4 miles per hour. At how many miles per hour must David cycle the last 9 miles in order to arrive just as work starts today?
10
12
14
16
18
