A man drove for 4 hours at a certain speed, he then doubled his speed and drove for another 3 hours. Although he covered 600 kilometers. At what speed did he drive for the last 3 hours?

A.
120km/hr 
B.
60km/hr 
C.
670km/hr 
D.
40km/hr
Correct Answer: Option B
Explanation
Speed = (frac{distance}{time})
let x represent the speed, d represent distance
x = (frac{d}{4})
d = 4x
2x = (frac{600 – d}{3})
6x = 600 – d
6x = 600 – 4x
10x = 600
x = (frac{600}{10})
= 60km/hr