For a simple approach, you will want to:

• open the output file
• loop through the list of input files
• open them
• loop through their contents
• average out their values
• print that to the output file

If you have more specific questions about the implementation, please ask them. We usually try to avoid giving out complete working solutions. [Of course, by the time I post this, someone might prove me wrong. But I'm willing to take that risk. :-)]

Update: indeed, m-rau proved me wrong.