If you can't answer that question and your business is a start-up, you are doing something wrong. Specifically, you aren't paying attention. You aren't paying attention to your cash flows, which will be vital the rest of your business life. You could be bleeding money getting your business up and running and unless you know your burn rate, you don't know how bad it really is. Have I convinced you that you need to know your burn rate? I hope so or some of you - actually many of you - will fail.
The burn rate is how fast you burn through cash in your business. If you borrowed money to start your business or if you had venture capitalists or angel investors back you, I guarantee you, they are concerned about your burn rate and they are going to be concerned if YOU are not concerned. Here's the article:What is Your Burn Rate and How is it Calculated?