Answer:
60 seconds
Step-by-step explanation:
Hi!
After she goes forward 5 feet (4 seconds), and the backwards 3 feet (another 4 seconds), the total distance forward she travelled is 2 feet. Then, she moved forward 2 feet in 8 seconds.
Then, as this is repeated, she moves forward 2 feet every 8 seconds. After 7 repetitions she travelled 14 feet, and 56 seconds elapsed. Then she goes forward 5 feet in 4 seconds, and she finaly reaches the end of the hallway
(14 +5 > 15)
So, in 60 seconds she reached the end of the hallway.