Astronomical observations of our milky way galaxy indicate that it has a mass of about 8. 0 • 1011 solar masses. A star orbiting on the galaxy’s periphery is about 6. 0 • 104 light years from its center.

17 a tourist was planning a trip by car for travelling at some constant speed for 2.5 hours. however, he found that if he increases the speed by 20 km/h he can cover the same distance in 30 minutes less time than it was planned. what was the planned speed of the tourist, in km/h?