The question says "How many feet did Dave go under the water", many people may think that the answer is 850 feet, but remember that, at the beginning, he was 250 feet above the sea level, meaning that in those 250 feet, he was not under the water level.
If you asking 2×7 then its 14 for each of them. 2 people for 7 instructors. But if your asking the probability its 28. I would like a little more question!!
