Brain Teasers
by Kiran Srinivas
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Book Summary Information
Author: Kiran SrinivasEdition: Paperback
Audio: English (Unknown); English (Original Language); English (Published)
Published: 2010-01-01
ISBN: 1885003994
Number of pages: 130
Publisher: Robert Reed Publishers
Book Reviews of Brain Teasers
Book Review: Mistake in Teaser #18
Summary: 5 Stars
The answer to Brain Teaser #18 is incorrect. !Spoiler Alert!
The answer should be 27000001.
The problem is that all the digital sums are not 55 as the answer key suggests. For example consider the pair:
(10 - 999,990)
Here the digital sum is 46!
In fact if you break it down only 450000 of the sums are 55, 45000 of the sums are 46, 4500 are 37, 450 are 28, 45 are 19 and 4 are 10.
So the method of solution is not viable because the above would be difficult to figure out (I used a computer).
Rather the proper analysis is to show that each place value contributes 100000 of each number for 1-999,999 (e.g, there are 100000 1's in the ones column, 100000 1's in the tens column, and so on)
So you get
600000 * 1 + 600000 *2 ... 600000 *9 =
600000 (1+2+3+4+5+6+7+8+9) =
600000 * 45 = 27,000,000
But you need to add 1 for 1,000,000
So the final answer is 27000001
This is also verified by the following Mathematica brute force program:
digitalSum[x_] := Total[Flatten[IntegerDigits[Table[i,{i,1,x}]]]]
digitalSum[1000000]
27000001
Don't know of any other mistakes and otherwise the book is excellent!!
Summary of Brain Teasers
To succeed in life, you must "think outside the box." This cliché is used to describe creative, non-standard approaches to problem solving. But how does one learn to "think outside the box?" Unfortunately, there is no simple answer. Author Kiran Srinivas believes that you can significantly improve your thinking abilities, and his book Brain Teasers will force you to think in ways you never have before. By sheer practice, you can increase your ability to think creatively.Many of the puzzles in this book will take hours, if not days, to solve, but be encouraged, a middle-school education is sufficient to answer almost all of the puzzles. There are many interview puzzles, and if you plan to interview at an investment bank, consulting firm, high-tech firm, or in any other related field, absorb what Brain Teasers has to offer, as it can be the deciding factor in getting that offer.Logic & Brain Teasers Books
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