You learned how to find an "average" (mean) way back in elementary school! (There are several different "averages". The mean is the one you first learned as an average.) Add up all the numbers and divide by 200.
If you know all that and your question is specifically about how to do it in Microsoft Excel, use "average".
Please do note if you're taking your data from a sample, you're calculating a sample mean. If you you're taking data from the whole population it is the population mean. The distinction is very important in statistics.