Answer: 15 yards per mistake.
Explanation:
Given : A football team had 4 big mistakes in a game.
i.e. the number of big mistakes done by the football team = 4
The total lost of yards because of the big mistakes done by football team = 60 yards
Now, the portion of team's yardage change per mistake is given by :-

Hence, the team's yardage changes by 15 yards per mistake.