We know that,
Julia can finish a 20-mile bike ride in 1.2 hours.
The distance Julia travels is 20 miles and the time she takes is 1.2 hours.
So, Julia's speed = = 16.67 mph
Katie can finish the same bike ride in 1.6 hours.
The distance Katie travels is 20 miles and the time she takes is 1.6 hours.
So, Katie's speed = = 12.5 mph
Now, to find how much faster Julia rides than Katie we subtract Katie's speed from Julia's speed.
So, 16.67 mph - 12.5 mph = 4.17 mph = 4.2 mph (approximately)
Thus, Julia rides 4.2 mph faster than Katie.