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A network of five cities, P, Q, R, S, and T, is connected by flights. The travel time (in hours) between directly connected cities is given in the table below. Assume that all connections are bidirectional. $$ \begin{array}{|c|c|} \hline \textbf{Connection} & \textbf{Travel Time (hours)} \\ \hline \text{P-Q} & 2 \\ \text{P-R} & 3 \\ \text{Q-R} & 1 \\ \text{Q-S} & 4 \\ \text{R-S} & 2 \\ \text{R-T} & 5 \\ \text{S-T} & 3 \\ \hline \end{array} $$ What is the minimum travel time required to go from City P to City T, visiting exactly two distinct intermediate cities? Assume no city can be visited more than once in a single journey. A) 7 hours B) 8 hours C) 9 hours D) 10 hours
The correct answer is B) 8 hours. **Step 1: Understand the Constraints and Objective** We need to find the minimum travel time from City P (start) to City T (destination). The key constraints are: 1. The journey must involve **exactly two distinct intermediate cities**. This means the path will be of the form $P \rightarrow X \rightarrow Y \rightarrow T$, where $X$ and $Y$ are intermediate cities chosen from {Q, R, S}, and $X \neq Y$. 2. No city can be visited more than once in a single journey. This implies that P, X, Y, T must all be distinct in the chosen path. **Step 2: List all possible ordered pairs of distinct intermediate cities ($X$, $Y$)** The available intermediate cities are Q, R, S. The possible ordered pairs ($X$, $Y$) are: * (Q, R) * (Q, S) * (R, Q) * (R, S) * (S, Q) * (S, R) **Step 3: Evaluate each possible path $P \rightarrow X \rightarrow Y \rightarrow T$ for validity and total travel time.** We will use the provided travel times, assuming connections are bidirectional. 1. **Path: $P \rightarrow Q \rightarrow R \rightarrow T$** * P-Q: $2$ hours * Q-R: $1$ hour * R-T: $5$ hours * Total time: $2 + 1 + 5 = 8$ hours. * Validity: P, Q, R, T are all distinct. This is a valid path. 2. **Path: $P \rightarrow Q \rightarrow S \rightarrow T$** * P-Q: $2$ hours * Q-S: $4$ hours * S-T: $3$ hours * Total time: $2 + 4 + 3 = 9$ hours. * Validity: P, Q, S, T are all distinct. This is a valid path. 3. **Path: $P \rightarrow R \rightarrow Q \rightarrow T$** * P-R: $3$ hours * R-Q: $1$ hour * Q-T: No direct connection exists between Q and T in the given table. * Validity: This path is invalid. 4. **Path: $P \rightarrow R \rightarrow S \rightarrow T$** * P-R: $3$ hours * R-S: $2$ hours * S-T: $3$ hours * Total time: $3 + 2 + 3 = 8$ hours. * Validity: P, R, S, T are all distinct. This is a valid path. 5. **Path: $P \rightarrow S \rightarrow Q \rightarrow T$** * P-S: No direct connection exists between P and S in the given table. * Validity: This path is invalid. 6. **Path: $P \rightarrow S \rightarrow R \rightarrow T$** * P-S: No direct connection exists between P and S in the given table. * Validity: This path is invalid. **Step 4: Identify the minimum travel time among valid paths** The valid paths and their total travel times are: * $P \rightarrow Q \rightarrow R \rightarrow T$: $8$ hours * $P \rightarrow Q \rightarrow S \rightarrow T$: $9$ hours * $P \rightarrow R \rightarrow S \rightarrow T$: $8$ hours The minimum travel time among these valid paths is $8$ hours. Therefore, the minimum travel time required is $8$ hours.
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