Answer
(a) $45.1$
(b) $45.06$
Work Step by Step
Given: $45.0649$
(a) Three significant digits:
All digits are significant.
Thus, rounding the given number to three significant digits is equivalent to rounding to the nearest tenths.
The tenth's digit is followed by a 6. Thus, round up the tenth's digit to $1$ and drop the the rest of the decimals to the right to obtain $45.1$
(b) Four significant digits
All digits are significant.
Thus, rounding the given number to four significant digits is equivalent to rounding to the nearest hundredths.
The hundredth's digit is followed by a 4. Thus, retain the hundredth's digit and drop the the rest of the decimals to the right to obtain $45.06$
