He traveled 5 Km in 20 minutes. There are three 20 minute periods in an hour, so in an hour he would travel three times as far, or 3 * 5 Km = 15 Km in an hour. That was his average speed, 15 Kph.
Using the standard formula, S = 5 Km / (1/3 hr) = (5 Km * 3) / (1 hr) = 15 Km/hr
When you divide by a fraction (1/3 hr), invert the fraction and multiply.