Katalin drove 300 miles on her vacation. She drove an average of 1.9 times faster on the second 150 miles of her trip than she did on the first 150 miles of her trip. Which expression represents the time she spent driving? Let x = her speed on the first half of the trip.
If Katalin drove at speed x on the first 150 miles, then she drove at speed 1.9x on the second 150 miles.
The time it took her to drive the first 150 miles is 150/x.
The time it took her to drive the second 150 miles is 150/(1.9x).
Therefore, the expression that represents the time Katalin spent driving is 150/x + 150/(1.9x). Answer: \boxed{150/x + 150/(1.9x)}.