I don't understand the language
y = -1/2x + 5 should be your equation because it is 5 ft above sea level and its eroding, or going down, 1 ft per 20 years. So every year, the sand dune erodes 1/20 ft more.
Hope this helps!
First, write out all the values:
40,41,41,45,48,48,49,49,49,50
Then to find the mean, you add all the values and divide by the number of values (there are 10 values)
(40+41+41+45+48+48+49+49+49+50)/10
460/10
=46
Hope this helps
You won't want to move the decimal if you're finding 1%.