It took him 12 mins to bike the trail because it implies that you bike faster than jogging so the time should be shorter hence bike twice as fast as jogging
It will take him 4 minutes to go around the trail with his bike once. He did 3 miles jogging in 24 minutes. So it took him 8 min per mile (24 /3 = 8). Since he bikes twice as fast as he jogs, it will take him half the time (8/2 = 4).