Leo needs to drive from boston to chicago. he planned to drive for 20 hours at an average speed of 50 mph. but drove 60 mph for the first 5 hours. during the remaining part of the trip, his average speed was 50 mph, how much earlier did he arrive in chicago than he originally planned?

He arrived 1 hour earlier.

The formula for distance in this case is
d=rt, where d is distance, r is rate (speed) and t is time.

The first leg of his trip, he traveled at 60 mph for 5 hours:
d = 60(5) = 300
He traveled 300 miles during the first part of his trip.

He had originally planned to travel 50 mph for 20 hours:
d = rt = 50(20) = 1000

His entire trip is 1000 miles.

The second part of the trip is then represented by
1000-300 = 50t
700 = 50t

Divide both sides by 50:
700/50 = 50t/50
14 = t

He traveled 5 hours for the first part and 14 hours the second part, for a total of 19 hours.
This is 20-19 = 1 hour earlier than he'd planned.