To solve this problem, we need to determine the speed of the bus given the time they meet and the speed of the car.

Step-by-Step Solution:

1. Calculate the time taken for the car and the bus to meet:

   They pass each other at 1620 hours, starting from 1320 hours. The time difference is: $1620 \, \text{hours} - 1320 \, \text{hours} = 4 \, \text{hours}$

2. Calculate the distance covered by the car in 4 hours:

   The car is travelling at a speed of 90 km/h. Therefore, the distance covered by the car in 4 hours is: $\text{Distance}_{\text{car}} = \text{Speed}_{\text{car}} \times \text{Time} = 90 \, \text{km/h} \times 4 \, \text{hours} = 360 \, \text{km}$

3. Determine the remaining distance covered by the bus:

   The total distance between the car and the bus was 510 km. The remaining distance, which is the distance covered by the bus, is: $\text{Distance}_{\text{bus}} = 510 \, \text{km} - 360 \, \text{km} = 150 \, \text{km}$

4. Calculate the speed of the bus:

   The bus also travels for 4 hours (since they meet after 4 hours). Therefore, the speed of the bus is: $\text{Speed}_{\text{bus}} = \frac{\text{Distance}_{\text{bus}}}{\text{Time}} = \frac{150 \, \text{km}}{4 \, \text{hours}} = 37.5 \, \text{km/h}$

Conclusion:

The speed of the bus is $37.5 \, \text{km/h}$.

Would you like details or have any questions? Here are 5 questions you can ask next:

1. How to solve problems involving relative speed?
2. Can you explain more about time-distance-speed relationships?
3. What are some other examples of relative motion problems?
4. How to deal with problems involving different units of speed and distance?
5. Can you solve a problem involving three moving objects?

Tip: Always ensure units are consistent when solving motion problems to avoid calculation errors.