A man drove for 4 hours at a certain speed; he then doubled his speed and drove for another 3 hours. Altogether he covered 600 km. At what speed did he drive for the last 3 hours?
Worked solution (try it first)
Let the first speed be v km/h.
Distance is speed times time, so the first part is 4v km and the second, at 2v for 3 hours, is 6v km.
Altogether 4v+6v=600, so 10v=600 and v=60.
The last 3 hours were at double speed: 2×60=120 km/h, option A.