To complete a job, it takes: Alice and Bob 2 hours, Alice and Charlie 3 hours, and Bob and Charlie 4 hours. How long will the job take if all 3 work together? Many, many US college students set up the equations incorrectly and get the wrong answer. Can you solve it? Watch the video to understand the correct method and answer.

*Small correction: At 2:19 I incorrectly said "four fifths" and meant to say "four point five."

0:00 Problem
1:00 Common mistake
3:14 Solution

My blog post for this video
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Sources
Toom, Andrei. "A Russian teacher in America." Journal of Mathematical Behavior 12.2 (1993): 117-139. See the "Tom, Dick, and Harry working together problem" (page 134 in text). [ Ссылка ]

Jerome Dancis, Associate Professor Emeritus, University of Maryland, Algebraic word problems. www.math.umd.edu/~jnd/Algebraic_word_problems.pdf

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