Explanation:
average speed = total distance travelled / total time travelled
time to travel the first 6km: 6 / 50 = 3/25 (h)
time to travel the next 6km: 6 / 90 = 1/15 (h)
[I think there's problem in the question 'cause 900km/h sounds impossible for normal person to travel in normal condition]
The total time: 3/25 + 1/15 = 14/75 (h)
Average speed over the 12 km drive will be:

That could be a comet, or any one of the billions of meteoroids
moving in a cloud that's actually the remains of a shattered comet.
If the ground is flat, and both bullets are released at the same time from the same height, then they both hit the ground at the same time.
The horizontal motion of the one from the gun has no effect on its vertical motion.
Fire Service Day
International Firefighters' Day (IFFD) is observed on May 4. It was instituted after a proposal was emailed out across the world on January 4, 1999 due to the deaths of five firefighters in tragic circumstances in a bushfire in Australia.