Question 633948
Mary travels from the city to visit her sister in the country. On the way, she travels 55km/hr. She stays with her sister for 20 hours before starting home. On the return trip, she travels 45 km/hr. She returned home a grand total of 41 hours after she left. How far away, in kilometers, is Mary's house from her sister's?

What I've managed to reason:
41 hours total-20 hours visiting=21 hours traveling
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The avg speed of a round trip = 2*55*45/(55+45) km/hr
= 49.5 km/hr
rt distance = r*t = 49.5*21 km
Each way = 49.5*21/2 = 519.75 km
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The avg speed of a round trip = 2*55*45/(55+45) km/hr
You can trust me on that, or I can prove it.