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?

77 times 5 is the total time the commuter train over the period of 5 days. 77*5=385.

71+74+78+78+x=385. 71+74+78+78=301. 385-301 is 84. So the fifth trip was 84 minutes long.

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.