Johnny's home is m miles from school. Johnny runs to school at a speed of 8 mph and then walks home from school at half that speed. How long does it take Johnny to walk home?
Johnny walks home at a speed of 8 mph / 2 = <<8/2=4>>4 mph.
It takes him m miles / 4 mph = <<m/4>>m/4 hours to walk home. Answer: \boxed{\frac{m}{4}\text{ hours}}.