Write the rules for rounding off numbers.

(N/A) In the measurement of physical quantities,the result of a calculation often represents uncertainty. To maintain consistency,the result should have the same number of significant digits as the data used in the calculation. The following rules are applied for rounding off:
$(1)$ If the digit to be dropped is less than $5$,the preceding digit is kept unchanged.
For example,$2.753$ rounded off to three significant digits becomes $2.75$.
$(2)$ If the digit to be dropped is greater than $5$,the preceding digit is increased by $1$.
For example,$5.86$ rounded off to two significant digits becomes $5.9$.
$(3)$ If the digit to be dropped is exactly $5$,the following conventions are used:
- If the preceding digit is even,the digit $5$ is simply dropped.
For example,$2.745$ rounded off to three significant digits becomes $2.74$.
- If the preceding digit is odd,it is increased by $1$.
For example,$2.735$ rounded off to three significant digits becomes $2.74$.