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Posted August 17, 2024 By Presh Talwalkar. Read about me, or email me.
This question has made the rounds and stumped many people.
Divide 40 by 1/2 and add 15. What do you get?
As usual, watch the video for a solution.
Divide 40 by 1/2 and add 15. What do you get?
Or keep reading. "All will be well if you use your mind for your decisions, and mind only your decisions." Since 2007, I have devoted my life to sharing the joy of game theory and mathematics. MindYourDecisions now has over 1,000 free articles with no ads thanks to community support! Help out and get early access to posts with a pledge on Patreon.
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Answer To Divide 40 by 1/2 and add 15. What do you get?
(Pretty much all posts are transcribed quickly after I make the videos for them–please let me know if there are any typos/errors and I will correct them, thanks).
A common mistake is to think “divide 40 by 1/2” means to divide 40 into half, which would be 20. Then add 15 and you get 35.
Another mistake is to think “divide 40 by 1/2 (of itself)”. So half of 40 is 20, and if you divide 40 by 1/2 of itself, that is 40 divided by 20 which is equal to 2. Then add 15 to get 17.
But “divide 40 by 1/2” actually means 40 ÷ (1/2) which is equal to 80. To understand why, imagine you had 40 apples and you eat 2 apples per day. How many days would it take to eat all the apples? This is the problem of 40 ÷ 2 = 20, so you eat the apples in 20 days. Imagine instead you ate 1/2 an apple per day. It clearly takes you 2 days to eat 1 apple, so it would take 80 days to eat all of the 40 apples. The problem translates into 40 ÷ (1/2), which as we reasoned would be 80.
So “divide 40 by 1/2” is 40 ÷ (1/2) = 80, and then you add 15 to get 95. This is without a doubt the correct answer to this question.
But funnily if you https://www.google.com/search?q=divide+40+by+1%2F2+plus+15&oq=divide+40+by+1%2F2+plus+15“>type it into Google the calculator adds 1/2 and 15 first, so you get 40/15.5 = 2.58 approximately.
So this simple question can even trick Google! But you’ll know the answer is 95.
And for once almost everyone agrees: a poll I did on YouTube had the result of 93 percent of people selecting the correct answer of 95.
References
Mirror
https://www.mirror.co.uk/news/weird-news/confusing-maths-question-people-scratching-33272565
YouTube poll
https://www.youtube.com/post/UgkxCI9nSfqBEObfpRHoCMzoMN_zUyo-1ZR-
Facebook posts
https://www.facebook.com/5.min.crafts.usa/posts/divide-40-by-12-and-add-15what-did-you-get/684471857117634/
https://www.facebook.com/lessonslearnedinlife/posts/divide-40-by-12and-add-15what-did-you-get-/7474944392577202/?_rdr
Google search (as of August 2024)
https://www.google.com/search?q=divide+40+by+1%2F2+plus+15&oq=divide+40+by+1%2F2+plus+15
Published by
PRESH TALWALKAR
I run the MindYourDecisions channel on YouTube, which has over 1 million subscribers and 200 million views. I am also the author of The Joy of Game Theory: An Introduction to Strategic Thinking, and several other books which are available on Amazon.
(As you might expect, the links for my books go to their listings on Amazon. As an Amazon Associate I earn from qualifying purchases. This does not affect the price you pay.)
By way of history, I started the Mind Your Decisions blog back in 2007 to share a bit of math, personal finance, personal thoughts, and game theory. It's been quite a journey! I thank everyone that has shared my work, and I am very grateful for coverage in the press, including the Shorty Awards, The Telegraph, Freakonomics, and many other popular outlets.
I studied Economics and Mathematics at Stanford University.
People often ask how I make the videos. Like many YouTubers I use popular software to prepare my videos. You can search for animation software tutorials on YouTube to learn how to make videos. Be prepared--animation is time consuming and software can be expensive!
Feel free to send me an email [emailprotected]. I get so many emails that I may not reply, but I save all suggestions for puzzles/video topics.
MY BOOKS
If you purchase through these links, I may be compensated for purchases made on Amazon. As an Amazon Associate I earn from qualifying purchases. This does not affect the price you pay.
Book ratings are from January 2023.
(US and worldwide links)
https://mindyourdecisions.com/blog/my-books
Mind Your Decisions is a compilation of 5 books:
(1) The Joy of Game Theory: An Introduction to Strategic Thinking
(2) 40 Paradoxes in Logic, Probability, and Game Theory
(3) The Irrationality Illusion: How To Make Smart Decisions And Overcome Bias
(4) The Best Mental Math Tricks
(5) Multiply Numbers By Drawing Lines
The Joy of Game Theory shows how you can use math to out-think your competition. (rated 4.3/5 stars on 290 reviews)
40 Paradoxes in Logic, Probability, and Game Theory contains thought-provoking and counter-intuitive results. (rated 4.2/5 stars on 54 reviews)
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 4.1/5 stars on 33 reviews)
The Best Mental Math Tricks teaches how you can look like a math genius by solving problems in your head (rated 4.3/5 stars on 116 reviews)
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 4.4/5 stars on 37 reviews)
Mind Your Puzzles is a collection of the three "Math Puzzles" books, volumes 1, 2, and 3. The puzzles topics include the mathematical subjects including geometry, probability, logic, and game theory.
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 112 reviews.
Math Puzzles Volume 2 is a sequel book with more great problems. (rated 4.2/5 stars on 33 reviews)
Math Puzzles Volume 3 is the third in the series. (rated 4.2/5 stars on 29 reviews)
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Teachers and students around the world often email me about the books. Since education can have such a huge impact, I try to make the ebooks available as widely as possible at as low a price as possible.
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