The answers to the sub-questions are:
- (a) Yes, we can use the normal distribution.
- (b) Yes, we can use the normal distribution.
- (c) If a random sample of size \( n \) is taken from any population, the distribution of the sample mean becomes normal as the sample size increases.
The probability that the sample mean is between 28 and 31 weeks is \( P = 0.5434 \).
Thus, the final answer is:
\[
\boxed{P = 0.5434}
\]