Rounding Question (manhattan books)

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.

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

great explanations. thank you!