Yes, for the first question you correct; at a rate of [tex] \frac{4.5mil}{30sec} [tex], one second of airtime would cost $150,000.
For the second, no you wouldn't be able to get 20 minutes of airtime with $3 million. Let's set up the equation; [tex] \frac{3mil}{xsec} = \frac{150000dollars}{1sec} \\ 3mil*1=150000*x \\ x = \frac{3mil}{150000} = 20 sec [tex] So at a rate of $150,000 per second, $3 million gets you 20 seconds of airtime.