Need a customized countdown script

I need a simple script that will countdown numbers at a non-regular pace. Example: There are only 10 spots left! Once the page loads - 5 seconds later the number drops to "9" 20 seconds after that the number drops to "8" 15 seconds after that the number drops to "7" 20 seconds later the number drops "6" 10 seconds later the number drops to "5" *** I need to be able to set the timer on when the numbers reduce each time. Should be very simple for somebody who knows what to do. Please advise?