William is a jockey. He can ride his horse for 6 hours a day. Over 6 days, he only used the maximum riding time twice. On two days he rode his horse for only 1.5 hours a day and half the maximum time for the next two days. How many hours did William ride his horse during those 6 days? Let's think first. Chain of thought:
William used two days to the limit, which means 6 * 2 = 12 hours of horse riding. For two days he rode a horse 1.5 a day, which makes up to 1.5 * 2 = 3 hours. For the next two days, he used only half of the maximum time which means 6 * 0.5 = 3 hours a day. That means 2 * 3 = 6 hours during these two days. So in total William rode his horse for 12 + 3 + 6 = 21 hours.
Therefore, the answer is 21.