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Carissa the chessplayer is a very, very slow walker. In fact, she walks at $100$ meters per hour when walking uphill, $120$ meters per hour when walking across flat ground, and $150$ meters per hour when walking downhill. One day, Carissa walks across Boston from a café to a boba shop, and then takes the same route in reverse to return to the café. What was Carissa's average speed during the entire round trip?

 Aug 22, 2023
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When the distance covered back and forth is the same, then:

 

Average speed ==Harmonic mean

 

Harmonic mean==3 / [100^-1  +  120^-1  +  150^-1] ==120 m/hour

 

Average speed ==120 meters/hour.

 Aug 22, 2023

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