A sonar device on a submarine sends an ultrasound signal into the ocean. The signal reflects off an underwater reef and is detected by the submarine's receiver exactly 2.4 seconds later. If the speed of sound in seawater is 1500 m/s, how far away is the reef from the submarine?

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• A 3600 m
• B 1800 m
• C 750 m
• D 1250 m

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Answer: B. 1800 m

The sound travels to the reef and back, so 2d = speed × time = 1500 × 2.4 = 3600 m. Therefore d = 1800 m.

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