A man travels at a rate of 25m/sec. If he travels for 10½hrs, how many kilometres has he covered?

Analyze the question: The speed is in metres per second (m/s), but the time is in hours and the answer must be in kilometres. So we convert the units first.

Concept: Distance = Speed × Time

Step 1: Convert the speed to km/h

• 25 m/s = 25 × 3600 m/h = 90,000 m/h
• 90,000 m = 90 km, so the speed is 90 km/h.

(Shortcut: to change m/s to km/h, multiply by 3.6. So 25 × 3.6 = 90 km/h.)

Step 2: Calculate the distance

• Time = 10½ hrs = 10.5 hrs
• Distance = 90 × 10.5 = 945 km

Check with seconds:

• 10.5 hrs = 10.5 × 3600 = 37,800 s
• Distance = 25 × 37,800 = 945,000 m = 945 km ✓

Answer: B. 945 km

Tip: Whenever units are mixed in a question, convert them first before you multiply. Remember: m/s → km/h means × 3.6, and km/h → m/s means ÷ 3.6. Many wrong options in exams, such as 262.5 here, come from forgetting this conversion.