A boat has a speed of 20 km/h in still water. The speed of the current is 5 km/h. How much time (in hours) will the boat take to travel 100 km downstream (with the current)?

Introduction / Context:
This question asks you to compute travel time when both the boat's own speed and the stream's speed are given. You must first determine the effective downstream speed and then apply the basic distance–speed–time relation.

Given Data / Assumptions:
• Boat speed in still water = 20 km/h.• Stream speed = 5 km/h.• Distance to be travelled downstream = 100 km.

Concept / Approach:
Downstream, the current helps the boat. So the downstream speed is the sum of the boat's speed in still water and the speed of the current. Once we know the effective speed, time taken is simply distance divided by speed.

Step-by-Step Solution:
Step 1: Downstream speed = 20 + 5 = 25 km/h.Step 2: Distance to be covered = 100 km.Step 3: Time = distance ÷ speed = 100 ÷ 25 hours.Step 4: 100 ÷ 25 = 4 hours.

Verification / Alternative check:
In 1 hour the boat travels 25 km downstream. In 4 hours it will cover 4 × 25 = 100 km, which matches the required distance, confirming that 4 hours is correct.

Why Other Options Are Wrong:
2 hrs or 3 hrs: These would correspond to speeds of 50 km/h or about 33.33 km/h, which are higher than the available downstream speed of 25 km/h.7 hrs: This would imply a downstream speed of less than 15 km/h, too slow for a 20 km/h boat helped by a 5 km/h current.

Common Pitfalls:
Some learners incorrectly subtract the stream speed even for downstream motion or forget to add the speeds. Always remember: downstream = boat + stream; upstream = boat − stream. Then apply the distance–speed–time formula accurately.

Final Answer:
The boat takes 4 hours to travel 100 km downstream.