## 3.19 Why and When HM

• When there are rates associated, say speed, and numerator is fixed.
• $$Speed, v = \frac{S}{t}$$; HM if S is fixed
• Example: A man travels 120 km the first day at 12 kph, the same distance at 10 kph on the 2nd day, and at 8 kph on the 3rd day. Find his average speed.