# Thread: Rounding off numbers question

1. ## Rounding off numbers question

OK Guys just a simple question but i can get it through my head.

I want to round off this number to 3 significant digit
18.45056065

Now as far as i know, the rules are
if the two digits following the cutoffpoint is exactly 50 the precedding figure
is not altered if it is an even number.

So following this rule the right answer should be 18.4 right?
The right answer on my book is 18.5. Please help me out!! I cant seem to get why im getting the wron answer! perhaps the book is wrong?

2. Originally Posted by jepal
OK Guys just a simple question but i can get it through my head.

I want to round off this number to 3 significant digit
18.45056065

Now as far as i know, the rules are
if the two digits following the cutoffpoint is exactly 50 the precedding figure
is not altered if it is an even number.

So following this rule the right answer should be 18.4 right?
The right answer on my book is 18.5. Please help me out!! I cant seem to get why im getting the wron answer! perhaps the book is wrong?

But maybe the book does not follow the rule. 'Cause in our Math class, we do not follow that rule, but in our Chem class, we do.

3. Originally Posted by reiward

But maybe the book does not follow the rule. 'Cause in our Math class, we do not follow that rule, but in our Chem class, we do.
its actually for a chem class maybe the book is wrong. I know this is a math forum, so i just assumed the rule would be the same. Any other input?

4. I do not understand your question. When rounding, if the digit to round is 0, 1, 2, 3 or 4, you round towards the bottom, otherwise you round towards the top.
In your case : 18.45056065, rounded to 3 s.f.
Therefore, you want to round off the first "5". Since it is a 5, it will be rounded towards the top, so the "4" becomes a "5". Hence, the correct answer is 18.5

5. Originally Posted by Bacterius
I do not understand your question. When rounding, if the digit to round is 0, 1, 2, 3 or 4, you round towards the bottom, otherwise you round towards the top.
In your case : 18.45056065, rounded to 3 s.f.
Therefore, you want to round off the first "5". Since it is a 5, it will be rounded towards the top, so the "4" becomes a "5". Hence, the correct answer is 18.5
There is actually a rule that says when rounding off an even number preceeding the digit "5", you do not add 1 to that number. E.g 18.45 becomes 18.4 ;

It is only applicable though in even numbers.