Answer:
Average speed 600 km/hr
Reduced to 400 km/hr
Step-by-step explanation:
30 mins = 0.5 hours
600/(x-200) - 600/x = 0.5
600x - 600x + 120000 = 0.5x² - 100x
0.5x² - 100x - 120000 = 0
x² - 200x - 240000 = 0
x² - 600x + 400x - 240000 = 0
x(x - 600) + 400(x - 600) = 0
(x + 400)(x - 600) = 0
x = 600