Now that "everyone" has a calculator that will give a result to six or eight (or more) figures, it is important that we know how to round the answer off correctly. The typical rule taught is that you round up with five or more and round down with four or less.

However, please do not rush off to your elementary school teacher and read 'em the riot act!

The problem lies in rounding "up" (increasing) the number that is followed by a 5. For example, numbers like 3.65 or 3.75, where you are to round off to the nearest tenth.

OK, let's see if I can explain this. When you round off, you change the value of the number, except if you round off a zero. Following the old rules, you can round a number down in value four times (rounding with one, two, three, four) compared to rounding it upwards five times (five, six, seven, eight, nine). Remember that "rounding off" a zero does not change the value of the number being rounded off.

Suppose you had a very large sample of numbers to round off. On average you would be changing values in the sample downwards 4/9ths of the time, compared to changing values in the sample upward 5/9ths of the time.

This means the average of the values AFTER rounding off would be greater than the average of the values BEFORE rounding.

This is not acceptable.

We can correct for this problem by rounding "off" (keeping the number the same) in fifty percent of the roundings-even numbers followed by a 5. Then, on average, the roundings "off" will cancel out the roundings "up."

The following rules dictate the manner in which numbers are to be rounded to the number of figures indicated. The first two rules are more-or-less the old ones. Rule three is the change in the old way.

When rounding, examine the figure following (i.e., to the right of) the figure that is to be last. This figure you are examining is the first figure to be dropped.

1. If it is less than 5, drop it and all the figures to the right of it.
2. If it is more than 5, increase by 1 the number to be rounded, that is, the preceeding figure.
3. If it is 5, round the number so that it will be even. Keep in mind that zero is considered to be even when rounding off.

Example #1 - Suppose you wish to round 62.5347 to four significant figures. Look at the fifth figure. It is a 4, a number less than 5. Therefore, you will simply drop every figure after the fourth, and the original number rounds off to 62.53.

Example #2 - Round 3.78721 to three significant figures. Look at the fourth figure. It is 7, a number greater than 5, so you round the original number up to 3.79.

Example #3 - Round 726.835 to five significant figures. Look at the sixth figure. It is a 5, so now you must look at the fifth figure also. That is a 3, which is an odd number, so you round the original number up to 726.84.

Example #4 - Round 24.8514 to three significant figures. Look at the fourth figure. It is a 5, so now you must also look at the third figure. It is 8, an even number, so you simply drop the 5 and the figures that follow it. The original number becomes 24.8.

When the value you intend to round off is a five, you MUST look at the previous value ALSO. If it is even, you round down. If it is odd, you round up. A common question is "Is zero considered odd or even?" The answer is even.

Here are some more examples of the "five rule." Round off at the five.

3.075

3.85

22.73541

0.00565

2.0495

This last one is tricky (at least for high schoolers being exposed to this stuff for the first time!). The nine rounds off to a ten (not a zero), so the correct answer is 2.050, NOT 2.05.

Would your teacher be so mean as to include problems like this one on a test? In the ChemTeam classroom, the sufferers (oops, I mean students) have learned to shout "YES" in unison to such easy questions.

Lastly, before we get to the problems. Students, when they learn this rule, like to apply it across the board. For example, in 2.0495, let's say we want to round off to the nearest 0.01. Many times, a student will answer 2.04. When asked to explain, the rule concerning five will be cited. However, the important number in this problem is the nine, so the rule is to round up and the correct answer is 2.05.

Round the following numbers as indicated.
To four figures:	To the nearest 0.1:	To nearest 0.01:	To the nearest whole number:
1) 2.16347 x 105	13) 3.64		25) 6.675		37) 56.912
2) 4.000574 x 106	14) 4.55		26) 0.4203		38) 3.4125
3) 3.682417		15) 7.250		27) 0.03062		39) 251.7817
4) 7.2518		16) 0.0865		28) 4.500		40) 112.511
5) 375.6523		17) 0.5182		29) 2.473		41) 63.541
6) 21.860051		18) 2.473		30) 7.555		42) 7.555

To two figures:		To one decimal place:	To the nearest 0.001:	Round off the farthest right digit
7) 3.512		19) 54.7421		31) 5.687524		43) 2.473
8) 25.631		20) 100.0925		32) 39.861214		44) 5.396
9) 40.523		21) 1.3511		33) 104.97055		45) 8.235
10) 2.751 x 108		22) 79.2588		34) 41.86632		46) 3.05
11) 3.9814 x 105	23) 0.9114		35) 0.03765		47) 8.25
12) 22.494		24) 0.2056		36) 0.0045		48) 8.65