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Rounding Question (manhattan books)

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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Rounding Question (manhattan books)

by kwhite » Fri Dec 23, 2011 12:15 am
I am using manhattan prep. I keep coming across questions such as this:

In the decimal, 2.4d7, d represents a digit from 0 to 9. If the value of the decimal rounded to the nearest tenth is less than 2.5, what are the possible values of d?

Answer= 0,1,2,3,4

I am under the impression that that the '7' would round up to 2.45->2.5 if d=4. Is the book wrong here? should i consider the thousandths decimal spot when rounding on the gmat?

thank you
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by GmatMathPro » Sat Dec 24, 2011 8:37 pm
kwhite wrote:I am using manhattan prep. I keep coming across questions such as this:

In the decimal, 2.4d7, d represents a digit from 0 to 9. If the value of the decimal rounded to the nearest tenth is less than 2.5, what are the possible values of d?

Answer= 0,1,2,3,4

I am under the impression that that the '7' would round up to 2.45->2.5 if d=4. Is the book wrong here? should i consider the thousandths decimal spot when rounding on the gmat?

thank you
The book is not wrong. 2.447 rounded to the nearest tenth is 2.4. When you round you should only look at the digit immediately to the right of the place value to which you're rounding.

What you're really trying to do is determine what the best approximation for 2.447 is: 2.4 or 2.5. 2.5-2.447=0.053, whereas 2.447-2.4=0.047. 2.4 is closer, so 2.4 is the better approximation. It's true that 2.45 rounds to 2.5, but 2.45 is not the number we're trying to approximate, it's just a number you got by doing an intermediate rounding of the original number.
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by smackmartine » Sun Dec 25, 2011 7:32 pm
The question actually tries to check a common misconception that many people have while rounding the decimal.
ROUND ONLY ONCE TO THE PLACE YOU ARE ASKED TO CONSIDERING ONLY IMMEDIATE DIGIT. (AS GmatMathPro mentioned )

MISCONCEPTION : ROUNDING MORE THAN ONCE
TAKEAWAY : ROUND ONCE
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It takes time and effort to explain, so if my comment helped you please press Thanks button :)
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by kwhite » Wed Feb 15, 2012 10:55 pm
great explanations. thank you!
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