#### Answer

a) 3985
b) 442.0
c) $1.860\times10^{5}$
d) $3.390\times10^{4}$
e) $6.321\times10^{4}$
f) $5.047\times10^{-4}$

#### Work Step by Step

a) 3984.6
All the digits are significant since they are not zero. So to express this number in four significant figures, one has to round it to 4 digits, which is
3985.
b) 442.04
All the digits are significant, since they are nonzero digits or zeros between nonzero numbers. So to round it to 4 significant figures, one has to round it to 4 digits, which is 442.0
c) 186,000
All the digits are significant, since they are nonzero digits or zeros at the end of a number. So to round it to 4 significant figures, one has to round it to 4 digits. This is only possible with the exponential notation, which is $1.860\times10^{5}$.
d) 33,900
All the digits are significant, since they are nonzero digits or zeros at the end of a number. So to round it to 4 significant figures, one has to round it to 4 digits. This is only possible with the exponential notation, which is $3.390\times10^{4}$.
e) $6.321\times10^{4}$
All the digits are significant, since they are nonzero digits. To round it to 4 significant figures, one has to round it to 4 digits. This is already the case, so the answer is $6.321\times10^{4}$.
f) $5.0472\times10^{-4}$
All the digits are significant, since they are nonzero digits or zeros between nonzero numbers. So to round it to 4 significant figures, one has to round it to 4 digits, which is $5.047\times10^{-4}$