3.
Given foci of (+8, 0) and difference of the focal radii = 6, find the equation of hyperbola.

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.