It takes Martha 30 minutes to walk uphill 1km from her home to school, but it takes her only 10 minutes to walk from school to home along the same route. What is her averagte speed, in km/hr, for the round trip?
+639 She walked 1 km to school and 1 km back home, her total distance is 1+1=2 km.
She spent 30 minutes walking to school and 10 minutes walking back home, her total time is 30+10=40 minutes = \(\frac{40}{60}=\frac{2}{3} hours\)
Therefore her average speed in km is $\(\frac{2}{\frac{2}{3}}=\boxed{\mathrm{(A)}\ 3}\)
