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."

My blog post for this video

https://wp.me/p6aMk-5h1

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). https://faculty.utrgv.edu/eleftherios.gkioulekas/OGS/Misc/ARUSSIAN.PDF

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

If you like my videos, you can support me at Patreon:

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"The Joy of Game Theory" shows how you can use math to out-think your competition. (rated 3.8/5 stars on 31 reviews)

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"Math Puzzles Volume 1" features classic brain teasers and riddles with complete solutions for problems in counting, geometry, probability, and game theory. Volume 1 is rated 4.4/5 stars on 13 reviews.

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"Math Puzzles Volume 2" is a sequel book with more great problems. (rated 5/5 stars on 3 reviews)

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"40 Paradoxes in Logic, Probability, and Game Theory" contains thought-provoking and counter-intuitive results. (rated 4.3/5 stars on 12 reviews)

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"The Best Mental Math Tricks" teaches how you can look like a math genius by solving problems in your head (rated 4.7/5 stars on 4 reviews)

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"Multiply Numbers By Drawing Lines" This book is a reference guide for my video that has over 1 million views on a geometric method to multiply numbers. (rated 5/5 stars on 3 reviews)

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

My blog post for this video

https://wp.me/p6aMk-5h1

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). https://faculty.utrgv.edu/eleftherios.gkioulekas/OGS/Misc/ARUSSIAN.PDF

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

If you like my videos, you can support me at Patreon:

https://www.patreon.com/mindyourdecisions

Connect on social media. I update each site when I have a new video or blog post, so you can follow me on whichever method is most convenient for you.

My Blog: https://mindyourdecisions.com/blog/

Twitter: https://twitter.com/preshtalwalkar

Facebook: https://www.facebook.com/pages/Mind-Your-Decisions/168446714965

Google+: https://plus.google.com/108336608566588374147/posts

Pinterest: https://www.pinterest.com/preshtalwalkar/

Tumblr: https://preshtalwalkar.tumblr.com/

Instagram: https://instagram.com/preshtalwalkar/

Patreon: https://www.patreon.com/mindyourdecisions

Newsletter (sent about 2 times a year): http://eepurl.com/KvS0r

If you buy from the links below I may receive a commission for sales. This has no effect on the price for you.

My Books

"The Joy of Game Theory" shows how you can use math to out-think your competition. (rated 3.8/5 stars on 31 reviews)

http://amzn.to/1uQvA20

"The Irrationality Illusion: How To Make Smart Decisions And Overcome Bias" is a handbook that explains the many ways we are biased about decision-making and offers techniques to make smart decisions. (rated 5/5 stars on 2 reviews)

http://amzn.to/1o3FaAg

"Math Puzzles Volume 1" features classic brain teasers and riddles with complete solutions for problems in counting, geometry, probability, and game theory. Volume 1 is rated 4.4/5 stars on 13 reviews.

http://amzn.to/1GhUUSH

"Math Puzzles Volume 2" is a sequel book with more great problems. (rated 5/5 stars on 3 reviews)

http://amzn.to/1NKbyCs

"Math Puzzles Volume 3" is the third in the series. (rated 3.8/5 stars on 4 reviews)

http://amzn.to/1NKbGlp

"40 Paradoxes in Logic, Probability, and Game Theory" contains thought-provoking and counter-intuitive results. (rated 4.3/5 stars on 12 reviews)

http://amzn.to/1LOCI4U

"The Best Mental Math Tricks" teaches how you can look like a math genius by solving problems in your head (rated 4.7/5 stars on 4 reviews)

http://amzn.to/18maAdo

"Multiply Numbers By Drawing Lines" This book is a reference guide for my video that has over 1 million views on a geometric method to multiply numbers. (rated 5/5 stars on 3 reviews)

http://amzn.to/XRm7M4

## Comments 4335 Comments

Alice gets 29% of the job done in one hour

Bob gets 21% of the job done in 1 hour

And Charly gets only 4% of the job in 1 hour.

1/(1/a+1/b+1/c) for a,b,c working together.

Using math to solve an estimation/psychological problem may not lead to the right answer but it can be used to put work efficiency into perspective.