12. A cross-country race car driver sets out on a 1.00 hour, 100.0 km race. At the halfway marker (50.0 km), the pit crew radios that the car had averaged a speed of only 80.0 km/h.
a) How long did it take the driver to travel the first 50.0 km? Show all work as demonstrated in
the lessons.
b) How fast must the driver drive over the remaining distance in order to average 100.0 km/h for

A.) Time = 0.625 hrs = 37.5 minutes

B.) Speed required = 133.33km/hr

Explanation:

Given the following :

Total race distance = 100km

Total time required = 1 hour = 60 minutes

Average speed after traveling 50km = 80km/hr

80km/hr : This means it will take one hour to cover a distance of 80km

Therefore, time taken to cove first 50km at that average speed equals :

1 hour = 80 km

t hours = 50km

80t = 50

t = 50/80

t = 5/8 hours

t = 0.625hours

t = 0.625 * 60 = 37.5minutes

B)

Average speed required to complete the race in 1 hour = 100km/hr

Time used to complete first 50km = 0.625 hour

Time remaining: (1 - 0.625) hour = 0.375

Speed required = Distance left / time left

Speed required = 50 / 0.375

Speed required = 133.33km/hr

the answer is b, have negligible mass