Bobby's cat runs at x mph. Bobby's dog runs twice as fast as his cat. How far can Bobby's cat run in 30 minutes?
In 30 minutes, there are 30/60 = <<30/60=0.5>>0.5 hours.
Bobby's cat can run at a distance of x*0.5 = <<x*0.5=0.5x>>0.5x miles in 30 minutes. Answer: \boxed{0.5x}.