During the first 18 min of a 60 min trip, a car has an average speed of 11 m s−1. What should be its average speed for the remaining 42 min so that its average speed for the entire trip is 21 m s−1?
(1) 25.3 m s−1
(2) 29.2 m s−1
(3) 31 m s−1
(4) 35.6 m s−1