Bobby's cat runs at x mph. Bobby's dog runs twice as fast as his cat. If the cat and the dog start running away from each other, how far away from each other will they be after 30 minutes?
Bobby's dog runs at 2*x mph.
In 30 minutes, Bobby's cat will run 30/60*x miles.
In 30 minutes, Bobby's dog will run 30/60*2*x = <<30/60*2*x=1/2*x>>1/2*x miles.
Together, Bobby's cat and dog will run (30/60*x) + (1/2*x) = 3/2*x miles.
They will be separated by 3/2*x miles after 30 minutes. Answer: \boxed{\frac{3}{2}x}.