2003 AMC 12A Problem 4

Problem 4 of 25EasierAlgebra

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 home along the same route. What is her average speed, in km/hr, for the round trip?

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Solution

Mary walks a total of

2

km in

30+10=40

minutes, which is

\dfrac23

hour. Her average speed is

2 \div \dfrac23 = 3

km/hr. Thus, the correct answer is A.

Concepts: distance rate and time · unit conversion · harmonic mean

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.