To find out how many more miles Oliver has to travel to reach his grandparents' house, you can subtract the distance he has already traveled from the total distance.
Total distance to grandparents' house: 228.4 miles
Distance traveled: 137.65 miles
Remaining distance = Total distance - Distance traveled
Remaining distance = 228.4 miles - 137.65 miles
Remaining distance = 90.75 miles
So, Oliver has to travel 90.75 miles to arrive at his grandparents' house.
The correct answer is 90.75.