sound travels in water at 1500 meters per second. how much time would it take for sound to travel at 6000 meters

Elena and her husband marc both drive to work. elena's car has a current mileage (total distance driven) of 7,000 and she drives 21,000 miles more each year. marc's car has a current mileage of 20,000 and he drives 11,000 miles more each year. will the mileages for the two cars ever be equal? explain.