+1348 It takes Mary $30$ minutes to walk uphill $1$ km from her home to school, but it takes her only $15$ minutes to walk from school to home along the same route. What is her average speed, in km/hr, for the round trip?
+129771 30 min = 1/2 hr
15 mn = 1/4 hr
Rate walking uphill = 1 km/(1/2) hr = 2 km/ hr
Rate walking downhill = 1 km / (1/4) hr = 4 km/hr
Avg rate when distance downhill = distance uphill = 2 ( product of the rates) / (sum of the rates) =
2 ( 2 * 4) / ( 2 + 4) = 16 / 6 = 8 / 3 = 2 1/3 km / hr
