A train travels from city `A` to city `B` with constant speed of `10 m s^(-1)` and returns back to city `A` with a constant speed of `20 m s^(-1)`. Find its average speed during its entire journey.

Explanation

To find the average speed of the train during its entire journey from city A to city B and back to city A, we can follow these steps:

### Step 1: Define the Distance
Let the distance between city A and city B be \( D \).

### Step 2: Calculate Time for Each Segment of the Journey
- **Time taken to travel from A to B**:
The speed from A to B is \( 10 \, \text{m/s} \).
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