A dolphin in `25^@C` sea water emits a sound directed toward the bottom of the ocean 150 m below. How much time passes before it hears an echo?
(Take speed cif sound in sea water at `25^@C = 1530` m/s)

Explanation

To solve the problem of how much time passes before the dolphin hears an echo, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Total Distance Traveled by the Sound:**
- The dolphin emits a sound that travels down to the bottom of the ocean and then reflects back to the dolphin. The distance to the bottom of the ocean is 150 meters.
- Therefore, the total distance the sound travels is:
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