The probability that a college freshman will graduate college is 0.5. Six friends enter college at the same time.
I'm afraid I might not fully understand your question - which is to say I'm not certain what you are asking. Nonetheless, I will try to make my best speculation.
If six friends enter college at the same time, one or more of them could graduate - though there is a 50% (0.5) probability that any particular individual will graduate. Let's add the numbers "of friends who could graduate": 1+2+3+4+5+6=21. Now, let's find the mean, or average: 21/6=3.5. Perhaps another way to do this would be to add the highest & lowest numbers, then divide by 2 or multiply by 0.5: 6+1=7, 7/2=3.5, 7x0.5=3.5.
Again, I may have misunderstood your question. If so, I apologize.