Computer Systems and Networking!? tell me how to work out the answer please!?

Question

How many bi-directional connections would be needed to connect 7 registers so that every register is connected directly to every other register? (i.e. as a fully-connected network)?

the answer is 21;

can someone please tell me why and how they worked it out? pleaseeee! asap!

Answer 1

The word "bi" means two, in bi-directional it means both ways, as opposed to "uni", if the question was uni-directional the answer is obviously 7 * 6 = 42, but in each "bi" it has two "uni"s so divide the answer by 2, here you go.

Or 6 + 5 + 4 + 3 + 2 + 1 would do as well, but it all hinges on the realization that it is bi-directional.