William used two days to the limit, which means 6 * 2 = <<6*2=12>>12 hours of horse riding.
For two days he rode a horse 1.5 a day, which makes up to 1.5 * 2 = <<1.5*2=3>>3 hours.
For the next two days, he used only half of the maximum time which means 6 * 0.5 = <<6*0.5=3>>3 hours a day.
That means 2 * 3 = <<2*3=6>>6 hours during these two days.
So in total William rode his horse for 12 + 3 + 6 = <<12+3+6=21>>21 hours.
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