A person travels from X to Y and back again. How long wil it take in travelling both the ways by bus?
A. It takes 21 hours in travelling form X to Y by train and returing by bus.
II. The distance between X and Y is 648 km.
C. A person can save 3 hours if he travels both the ways by train as compared to travelling by bus on the one side and returning by train.

Options

A. Only A and C together
B. B and either A or C
C. Any two of them
D. All statements are necessary

Explanation

To solve the problem, we need to determine how long it will take for a person to travel both ways from X to Y and back again by bus. We will analyze the provided statements step by step.

### Step-by-Step Solution:

1. **Understanding the Journey**:
- The person travels from X to Y and then returns from Y to X. We need to find the total time taken for this round trip by bus.

2. **Analyzing Statement A**:
- Statement A states that it takes 21 hours to travel from X to Y by train and return by bus.
- Let’s denote the time taken to travel from X to Y by train as \( T_{train} \) and the time taken to return from Y to X by bus as \( T_{bus} \).
- From Statement A, we have:
\[ T_{train} + T_{bus} = 21 \quad \text{(1)} \]

3. **Analyzing Statement B**:
- Statement B states that the distance between X and Y is 648 km.
- This statement alone does not provide any information about the time taken by bus or train.

4. **Analyzing Statement C**:
- Statement C states that a person can save 3 hours if he travels both ways by train compared to traveling one way by bus and returning by train.
- If the person travels both ways by train, the time taken would be \( 2 \times T_{train} \).
- The time taken for one way by bus and returning by train would be \( T_{bus} + T_{train} \).
- According to Statement C:
\[ 2 \times T_{train} = T_{bus} + T_{train} - 3 \quad \text{(2)} \]

5. **Setting Up the Equations**:
- From Equation (1), we can express \( T_{bus} \) in terms of \( T_{train} \):
\[ T_{bus} = 21 - T_{train} \quad \text{(3)} \]
- Substitute Equation (3) into Equation (2):
\[ 2 \times T_{train} = (21 - T_{train}) + T_{train} - 3 \]
\[ 2 \times T_{train} = 21 - 3 \]
\[ 2 \times T_{train} = 18 \]
\[ T_{train} = 9 \quad \text{(4)} \]

6. **Finding \( T_{bus} \)**:
- Substitute \( T_{train} = 9 \) back into Equation (3):
\[ T_{bus} = 21 - 9 = 12 \quad \text{(5)} \]

7. **Calculating Total Time for Both Ways by Bus**:
- The total time taken to travel both ways by bus is:
\[ 2 \times T_{bus} = 2 \times 12 = 24 \text{ hours} \]

### Conclusion:
The total time taken for the person to travel both ways by bus is **24 hours**.