Pablo can drive 6 times as fast as Carlos can ride his bicycle. If it takes Carlos 4 hours longer than Pablo to travel 24 miles, how fast can Carlos ride his bicycle?
Answer:
Let Carlos bike speed be x mph
Pablo's car speed is 6*x mph

To travel 24 miles, time taken by Carlos = 24/x
To travel 24 miles, time taken by Carlos = 24/6*x = 4/x

Difference between the 2 times is given as 4 hours.
So 

 

 or 

Speed of bike = 5 mph
Speed of car = 5*6 = 30 mph

Check for correctness: 
Time taken by bike = 24/5 = 4.8 hours
Time taken by car = 24/30 = 0.8 hours
Difference = 4.8 - 0.8 = 4 hours - correct!

:)