He went around 24 times and 4 times is a mile so 24/4 will tell us how many miles he biked. He biked 6 miles in thirty minutes. The formula distance = rate(time) will tell you how fast he is going.
6 = r(1/2) divide each side by 1/2 (The time has to be in hours)
12 = r
So he was going 12 miles an hour.