The average speed would be 600 km/hr.
Step-by-step explanation:
Let the original speed be 'x'
Let the decreased speed be 'x-200'
Distance = 600 km
Time increased by 30 minutes
According to question, it becomes,
[tex]\dfrac{600}{x-200}-\dfrac{600}{x}=\dfrac{30}{60}\\\\\dfrac{x-(x-200)}{x^2-200x}=\dfrac{1}{2\times 600}\\\\\dfrac{200}{x^2-200x}=\dfrac{1}{1200}\\\\240000=x^2-200x\\\\x^2-200x-240000=0\\\\x=600,-400[/tex]
So, The average speed would be 600 km/hr.