The conventional rules are:
(i) If the insignificant digit to be dropped is more than 5, the preceeding digit is increased by 1, but if it is less than five, then preceeding digits is not changed, e.g., 1.748 is rounded off to 3 significant figures as 1.75 and 1.742 as 1.74.
(ii) If the insignificant digit to be dropped is 5, then this digit is simply dropped if the preceeding digit is increased by 1.
E.g., the number 1.845 rounded off to three significant digits is 1.84 but for number 1.875 it is 1.88.
