What Peter says in Solution 1 is true, but doesn't really address the problems you are having, and will have with the way you are doing things.
Start by not using "named" variables to hold the data you read from the user: use an array instead:
for (i = 0; i < 5; i++)
Then finding the largest, or smallest, sum and average are all just very similar (and very simple) loops:
sum = 0;
for(i = 0; < < 5; i++)
sum += data[i];
And if your task changes to want ten numbers instead of five the changes become trivial.
Give it a try: it may seem more complex, but it really, really isn't once you get your head around it!