Exercise 35 Page 248

In order to solve the problem we can graph both functions and find where they intersect.

We can see that the first time the functions intersect is around $t=18.$ However, while your friend has already touched the fence and is heading back, you didn't get there yet. That is why we have to look at the second intersection at $t=30$ when you both are heading the same direction. This is the time you catch up to your friend.