count 'em ...
or think ...
2 computers ... 1 wire
3 computers ... 3 wires
4 computers ... 6 wires
5 computers ... 10 wires
see the pattern?
Yes, the sum of numbers up to n is (n*(n-1))/2; therefore, for the n=8, the answer is 56/2 = 28. We need one fewer wire, since we are not connecting a computer to itself, as explained in the answer already provided. We need that for all 8 computers, considered as a pair each time. Therefore 8*7 / 2 is both logical and correct.
I reiterate what was posted: we need one fewer wire. I added that we are NOT connecting a computer to itself, as you seem to imply the opposite.
If one draws out the connections, they come to 28. BUT THE LOGICAL ANSWER GIVEN, THAT WE NEED ONE FEWER WIRE FOR ALL 8 CONNECTIONS RESULTS IN THE FORMULA I GAVE, N(N-1)/2.
What part of that do you have a problem with, Plato?