17. A dolphin descends 12 feet from the surface of the ocean and then ascends 7 feet. Then it descends 15 feet and repeats this descent three more times. How many feet below the surface is the dolphin then?

To solve how far below the surface the dolphin ends up, we start with its initial descent of 12 feet, placing it at -12 feet (12 feet below the surface). Then it ascends 7 feet, bringing it to -5 feet (-12 + 7 = -5 feet). Next, it descends 15 feet, which brings it to -20 feet (-5 - 15 = -20 feet). Now, it repeats this descent three more times. Each descent of 15 feet, across three repetitions, will take it down another 45 feet (15 x 3 = 45). Adding this to -20 feet: -20 - 45 = -65 feet. Therefore, the dolphin ends up 65 feet below the surface.