Final answer:
Santiago traveled 180 miles before falling asleep on the road trip.
Step-by-step explanation:
To find out how many miles Santiago traveled, we first need to calculate 45% of the total length of the trip.
To do this, we multiply 400 miles (the total length of the trip) by 0.45 (which represents 45% as a decimal).
400 miles x 0.45
= 180 miles.
Therefore, Santiago traveled 180 miles before falling asleep.