At a speed of 50km/h, the time to complete is : 

\(t=\dfrac{s}{t}=\dfrac{900}{50}=18\left(hour\right)\)

Change : 900 km = 900. 0,62137 = 559.233 miles 

So , the velocity calculated by miles/hour is : \(\dfrac{599.233}{18}=33.29.....\approx33.291\)  (miles/hour)

With the time complete in 3 hours, the velocity is : \(\dfrac{599.233}{3}=199.744\) (miles/hour)

Hence, there are 199.744 - 33.291 = 166.453 (miles/hour) faster you must average while driving.

I don't very good at English

But I will study English much

Too can do some hard questions