we are asked in the problem to enumerate three numbers that are greater than 20 and have a factor of 9. In this case, we multiply 9 by numbers greater than 2, that results to numbers like 27, 36 and 45. There are other infinite numbers like 54, 63, 72, 81, 90, 99, etc.
780 feet because 200 feet below sea level means -200 and 580 feet above sea level mean +580 so if you add 580 and the 200 you get the distance between the two points.
