For example, if we are dividing a 20 centimeter wire into 3 equal pieces, we would divide 20 by 3 to get the length of each piece. The 6 repeats forever. We round to some usually pre-determined number of digits or decimal places. Sometimes rounding is the result of an approximation. We will round many of our answers in science because the numbers will often be reporting measurements .
Numbers representing measurements are only as accurate as the device used for measuring.
For example, we could use a standard meter stick marked off in centimeters to measure the length of a wire as 15 cm. If sometime later we cut the wire in pieces, reporting the size of a piece of the wire to 9 or 10 decimal places would not make sense. One other operation which you may or may not be familiar with is often used in chemistry. Absolute value is represented as a pair of upright lines around whatever expression or number is supposed to be treated that way. The absolute value of a number expresses its distance from zero, regardless of sign.