I need help with my practice question. Can anyone help and explain?

Question

quantity of movies- 0 1 2 3 4 5

Total utility of movies- 0 _ _ _ 99.5 101.5

Marginal Utility of movies _ 48 27 17 _ _

Answer 1

Quantity of movies 0..1....2.....3......4........5

Total utility               0..48..75...92..99.5...101.5

Marginal utility         0, 48, 27,  17....7.5....2.0

At quantity 1 marginal utility is 48, so total utility is 48.

At quantity 2 marginal utility is 27. So add 27 to total utility before of 48, you get total utility from 2 movies as 48+27= 75.

Add marginal utility of the 3rd movie to total utility of 2 movies you get total utilty of 3 movies as 75+17=92

Similarly the marginal utility from the 5th unit is the difference between total utility of 5 movies and total utility of 4 movies= 101.5-99.5= 2.0

Total utility from 4 movies is 99.5 and tital utilities from 3 movies is 92, so you get the marginal utility of the 4 as the difference 99.5-92= 7.5

Answer 2

...0...1....2....3 ...4........5

...0..48..75..92 .99.5..101.5

...X..48..27..17 ..7.5.....2

You have total utility line 0 _ _ _ 99.5 101.5

So check marginal utility - each one gives extra to total utility so 0+48=48 now it becomes 0 48 _ _ 99.5 101.5

next 48+27=75 now it becomes 0 48 75 _ 99.5 101.5

next 75+ 17=92 now it becomes 0 48 75 92 99.5 101.5

Opposite way marginal utility line: _ 48 27 17 _ _

Total utility changes from 99.5 to 101.5, so 101.5-99.5=+2

now it becomes _ 48 27 17 _ 2

next 99.5-92=7.5

now it becomes _ 48 27 17 7.5 2

X means that there no any marginal utility for zero quantity of movies. So it becomes X 48 27 17 7.5 2