Two aeroplanes start from the same place in opposite directions one goes towards east at 320 km/h and the other goes towards west at 400 km/h what time will they take to be 720 km apart?

Options

A. 4 h
B. 3h
C. 1 h
D. 1.5 h

Answer

To solve the problem of how long it will take for the two aeroplanes to be 720 km apart, we can follow these steps:

### Step-by-Step Solution

1. **Identify the speeds of the aeroplanes**:
- The first aeroplane is flying towards the east at a speed of 320 km/h.
- The second aeroplane is flying towards the west at a speed of 400 km/h.

2. **Calculate the relative speed**:
- Since the aeroplanes are moving in opposite directions, we can find their relative speed by adding their speeds together.
- Relative speed = Speed of first aeroplane + Speed of second aeroplane
- Relative speed = 320 km/h + 400 km/h = 720 km/h.

3. **Determine the distance to be covered**:
- The distance that we want the aeroplanes to be apart is 720 km.

4. **Use the formula for time**:
- The formula to calculate time when distance and speed are known is:
\[ \text{Time} = \frac{\text{Distance}}{\text{Speed}} \]
- Substituting the values we have:
\[ \text{Time} = \frac{720 \text{ km}}{720 \text{ km/h}} = 1 \text{ hour}. \]

### Final Answer
The two aeroplanes will take **1 hour** to be 720 km apart.

---