During the first 18 minutes of a 1.0-hour trip, a car has an average speed of 11 m/s. what must the average speed of the car be during the last 42 minutes of the trip be if the car is to have an average speed of 21 m/s for the entire trip?

The average speed is calculate by dividing the total distance covered in the travel by the total time it takes for the trip. The total time it takes for the car to travel is 1 hour which is also equal to 3600s. 

To answer the question, we let x be the average speed during the last 42 minutes. The equation that would best help us in solving this problem is,

 d =  (11 m/s)(18 min)(60 s/1 min) + (x)(42 min)(60 s/1 min) = 2520x + 11880 

t = 3600 s

average speed = 21 m/s = (2520x + 11880) / 3600

The value of x from the equation is 25.28 m/s.