Answer:
Option D is the correct answer.
Step-by-step explanation:
Let x be the one way distance and t be the time out.
Time taken for back = t-1
We have plane's average speed out was 300 mph and average speed on the way back was 350 mph.
That is
Dividing both equations
Time taken for back = t-1 = 7-1 = 6 hours.
Total time taken = 7 + 6 = 13 hours
Option D is the correct answer.