Help with exponential growth and modelling?

A certain type of bacteria replicates at a rate of 160% every four hours, but a hole in the dish allows 50,000 bacteria to escape every hour. How long does it take for the amount of bacteria to reach 10^12 if the initial amount of bacteria is 1 million? Using a spreadsheet, develop a model for the growth of this particular strain of bacteria.

I'd just like to know to do this, like how to get the amount of bacteria at time t=4. I've been stuck on this for a while now. (Headbang)

EDIT: I actually know the answer to this, but I don't know how to do it.