A bus covers 400 km at 20 km/h. How much time does it take to cover this distance?

Introduction / Context:
Time is distance divided by speed for uniform motion. The numbers lead to an exact integer solution.

Given Data / Assumptions:

• Distance = 400 km.
• Speed = 20 km/h.

Concept / Approach:
Use time = distance / speed.

Step-by-Step Solution:
Time = 400 / 20 = 20 h.

Verification / Alternative check:
Check: 20 km/h for 20 h yields 400 km.

Why Other Options Are Wrong:
25 h would imply 16 km/h; 5 h implies 80 km/h; 21 h implies ~19.05 km/h, none matching 20 km/h.

Common Pitfalls:
Inverting the ratio and mistakenly multiplying speed by time instead of dividing distance by speed when solving for time.

Final Answer:
20 h