(a) To find the probability that a restaurant is in the Northeast given that it is located in a city with a population over 500,000, use the formula for conditional probability: \( P(A|B) = \frac{P(A \cap B)}{P(B)} \). Here, \( A \) is the event that the restaurant is in the Northeast, and \( B \) is the event that the restaurant is in a city with a population over 500,000.
(b) To find the probability that a restaurant is in a city with a population under 50,000 given that it is located in the Southeast, use the same conditional probability formula. Here, \( A \) is the event that the restaurant is in a city with a population under 50,000, and \( B \) is the event that the restaurant is in the Southeast.
(c) To find the probability that a restaurant is in a city with a population of 500,000 or less given that it is located in the Southwest, use the conditional probability formula. Here, \( A \) is the event that the restaurant is in a city with a population of 500,000 or less, and \( B \) is the event that the restaurant is in the Southwest.
To find the probability that a restaurant is in the Northeast given that it is located in a city with a population over 500,000, we use the formula for conditional probability:
\[
P(\text{NE} | \text{over } 500k) = \frac{P(\text{NE} \cap \text{over } 500k)}{P(\text{over } 500k)}
\]
From the data, we have:
- \( P(\text{NE} \cap \text{over } 500k) = 56 \)
- \( P(\text{over } 500k) = 247 \)
Thus,
\[
P(\text{NE} | \text{over } 500k) = \frac{56}{247} \approx 0.2267
\]
Next, we calculate the probability that a restaurant is in a city with a population under 50,000 given that it is located in the Southeast:
\[
P(\text{under } 50k | \text{SE}) = \frac{P(\text{under } 50k \cap \text{SE})}{P(\text{SE})}
\]
From the data, we have:
- \( P(\text{under } 50k \cap \text{SE}) = 0 \)
- \( P(\text{SE}) = 128 \)
Thus,
\[
P(\text{under } 50k | \text{SE}) = \frac{0}{128} = 0.0
\]
Finally, we calculate the probability that a restaurant is in a city with a population of 500,000 or less given that it is located in the Southwest:
\[
P(\text{500k or less} | \text{SW}) = \frac{P(\text{500k or less} \cap \text{SW})}{P(\text{SW})}
\]
From the data, we have:
- \( P(\text{500k or less} \cap \text{SW}) = 13 \)
- \( P(\text{SW}) = 83 \)
Thus,
\[
P(\text{500k or less} | \text{SW}) = \frac{13}{83} \approx 0.1566
\]
The probabilities are as follows:
- (a) \( P(\text{NE} | \text{over } 500k) \approx 0.2267 \)
- (b) \( P(\text{under } 50k | \text{SE}) = 0.0000 \)
- (c) \( P(\text{500k or less} | \text{SW}) \approx 0.1566 \)
Thus, the final answers are:
\[
\boxed{(a) \, 0.2267, \, (b) \, 0.0000, \, (c) \, 0.1566}
\]