Answer :

The film would last until 12:15. I simply added on the one hour and fifty five minutes to find the answer.
11:20 A.M. is the start time.

1 hour later is 12:20 P.M.

Imagine 12:20 as a number instead of a time 1, 220 
Now add 50 to 220 and the sum is 270.
Add 5 to 1, 270 to get 1,275.
Now the time is 12:75 so subtract 60 minutes in a hour from 75 to get 15 minutes.
So the finished time of the film is 1:15 P.M.
Hope this helps! #UrWelcome