Aroller coaster starts from a deck at an elevation of 20 feet above the ground. on the first hill it climbs 83 feet and then drops 42 feet. on the second hill the coaster climbs 128 feet and then drops 90 feet. how far above the ground is the coaster after the second hill?

Initially started 20 feet from the ground . 
Went uphill 83 feet. 
So: 20 + 83 = 103. The coaster is 103 ft above at this point. 

Went down 42 feet 
So: 103 - 42 = 61. 

Went up 128 feet
So: 61 + 128 = 189

Finally, went down 90 feet
So: 189 - 90 = 99

99 ft above ground.

I would say 99 feet.  An elevation of 20 feet above the ground is the starting point.  Then as the coaster rises 83 feet, you add 20 + 83 = 103 feet.  Then it drops 42 feet.  103 - 42 = 61 feet.  Then it rises 128 feet on the second hill.  61 + 128 = 189 feet.  Finally, it drops 90 feet.  189 - 90 = 99 feet above the ground after the second hill.

Think of it as five or above give it a shove. change the 5 or above to the right to a zero and add one to the number on the left.