A man walks a certain distance at 5 km/h and runs back at 15 km/h. What is his average speed for the entire journey? MCQ with Answer and Explanation

Explanation:
Let original speed be s. 450/s - 450/(s+5) = 1 => 450(s + 5 - s) = s(s + 5) => 2250 = s(s + 5). Solving s^2 + 5s - 2250 = 0 gives (s - 45)(s + 50) = 0. Since speed is positive, s = 45 km/h.

Explanation:
Speed of the second train = 300 / 3 = 100 km/h. Speed ratio is 4:5, so 5 parts = 100 km/h => 1 part = 20 km/h. Speed of the first train = 4 parts = 4 * 20 = 80 km/h.

Explanation:
Speed = 5 km/h = 5000 meters / 60 minutes = 83.33 meters per minute. Distance in 12 minutes = 83.33 * 12 = 1000 meters.