Please use
code tags in the future, so your code is readable.
This statement doesn't have any effect because you never store the result anywhere. You can eliminate this from the two spots in your program.
Unfortunately, you don't have the right code, at least not according to the requirements. You are supposed to compute integers between n and m that are divisible by k. That is, integers for which x/k has no remainder, or x%k is zero (% is the modulus or remainder operator). Instead, you compute the average of all integers from m to n inclusive. But there's even a problem with that code, since you just do sum/k which is integer division, and gives incorrect results.
So there are equivalents to scanf and printf that work with files in general, and not just the standard input and output "files" (keyboard and monitor/terminal). You should look at fopen, fscanf, fprintf and fclose.
Actually, your english is pretty darn good. I had no trouble following what you said.