RRB NTPC CBT I 16 Jan 2021 Shift 1 Paper

P, Q and R are on a trip by a car. P drives during the first hour at an average speed of 40 km/h. Q drives during the next 2 hours at an average speed of 50 km/h. R drives for the next 3 hours at an average speed of 60km/h. If they reached their destination after exactly 6 hours, then find their mean speed approximately