Answer
(a) $9.999$
(b) $9.998$
Work Step by Step
RECALL:
(1) To round-off a number, check the digit immediately to the right of the round-off place. If the digit is 5 or higher, round up the value, otherwise, retain the value.
Examples:
Round-off to two decimal places:
$23.546 \longrightarrow 23.55
\\23.544 \longrightarrow 23.54$
(2) To truncate a number, drop all digits to the right of the specific truncation place.
Examples:
Truncate to two decimal places:
$23.546 \longrightarrow 23.54
\\23.544 \longrightarrow 23.54$
Thus, for the given number:
(a) Rounded-off to three decimal places:
$9.99\underline{8}\color{red}{5} \longrightarrow 9.999$
(b) Truncated to three decimal places:
$9.99\underline{8}\color{red}{5} \longrightarrow 9.998$
