Now all are safely across the river. You have two ropes. Each takes exactly 60 minutes to burn. They are made of different material so even though they take the same amount of time to burn, they burn at separate rates. In addition, each rope burns inconsistently so that you cannot use distance along the rope as a measure of time.
T ake one rope and light it at both ends and light one end of the other rope.
When the first rope finishes burning, light the other end of the remaining rope. A man comes home from work by train every day. He gets to the train station at 5 pm and from there his dog picks him up in the car. One day this man got to the station at 4 pm and decided to start walking and meet his dog somewhere between the train station and home. That day, they got home 20 minutes earlier.