what is 124.5 when rounding to the 3rd significant figure?
There are some deviations when five is concerned and you should ask your professor what procedure s/he prefers; however, the one I see most often is to round the five to the even number. Thus, 124.5 means we round to 124 to three s.f. 123.5 would be rounded to 124.
http://www.chemteam.info/SigFigs/Rounding.html
Most would round to 125.
A better question is rounding 123.5 . Most would round that to 124, but professinal scientific rounders wouldn't. If one always rounds up on halfway (5), then you round up more than down (down on 1,2,3,4, up on 5,6,7,8,9)
So professiona science rounders round up to the nearest even, or nearest odd.
A nearest even rounder would round
13.5 to 14
12.5 to 12
12.4 to 12
13.2 to 13
13.6 to 14
A nearest odd rounder would round
13.5 to 13
12.5 to 13
12.4 to 12
13.4 to 12
13.6 to 14
Nearly all I know are even rounders in chemistry, and in physics. There are also statistical methods to round.
http://en.wikipedia.org/wiki/Rounding
To round a number to the third significant figure, you need to look at the digit in the fourth place and round the number accordingly.
In the given number 124.5, the third significant figure is the digit 4.
To determine whether to round up or round down, we look at the digit in the fourth place, which is 5 in this case. Since 5 is greater than or equal to 5, we round up.
Therefore, when rounding 124.5 to the third significant figure, it would be rounded up to 125.