It takes Mary 30 minutes to walk uphill 1 km from her home to school, but it takes her only 10 minutes to walk from school to her home along the same route. What is her average speed, in km/hr, for the round trip?

uphill speed=33 1/3 km/h

downhill speed=100km/h

use harmonic mean=0.02999100269*2=0.59 aproximately

Average speed==Total distance / Total time

Average speed ==[2 x 1 km] /[30/60 + 10/60]

Average speed ==2 km /[2/3]==2 x 3 / 2 ==3 km/h - her average speed back and forth.