Ask Mar­i­lyn

Albuquerque Journal - Parade - - WHAT AMERICA EATS - By Mar­i­lyn vos Sa­vant

Say each of four towns is lo­cated at the cor­ner of a square. A mo­torist starts from town A to make a round trip at the fol­low­ing speeds: AB = 30 mph, BC = 40 mph, CD = 50 mph and DA = 60 mph. What is the aver­age speed of the round trip?

Try fig­ur­ing it out your­selves, read­ers. The an­swer ap­pears in the fol­low­ing para­graph.

No, the aver­age speed is not 45 mph. Huh? Con­sider this ex­am­ple: Say the square is 60 miles on each side. The mo­torist takes 2 hours to drive AB, 1.5 hours for BC, 1.2 hours for CD and 1 hour for DA. The to­tal dis­tance is 240 miles, and the driv­ing time is 5.7 hours. That’s about 42 mph. Send ques­tions to

mar­i­lyn pa­

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