Over a period of five days, the average time of a commuter train trip from one town to the city was 77 minutes. one trip was 71 minutes, another was 74, and two each were 78 minutes. how many minutes long was the fifth trip?

Average time of a commuter train trip from one town to the city = 77 minutes
Time taken by 1 trip = 71 minutes
Time taken by another trip = 74 minutes
Time taken by 3rd and 4th trip each = 78 minutes
Let us assume the time taken during the 5th trip = x
Then
(71 + 74 + 78 +78 +x)/5 = 77
301 + x = 77 * 5
301 + x = 385
x = 385 - 301
   = 84 minutes
So the length of the fifth trip was 84 minutes. I hope the procedure is clear enough for you to understand.