text: n*(n-1/2) project managment  formula question? could you please explaing why the answer is 18.  the project manager has 4 people reporting to him. three more people are added to the project. how many communication channels were added to the project?  the formula is n*(n-1)/2   first calculate the number of communication channels based on the existing team 4 and the project mang. =  5. that is 10 communication channels. next, calculate the numbre of communication channels with the new people added. this is 28 communication channels. finally, subtract the difference (28-10=18)  i still don't understand how the answer is 18 let call a, b, c and d the 4 first people e,f,g the three others and m the manager. the communication channels are : a-b, a-c, a-d, b-c, b-d, c-d, a-m, b-m, c-m and d-m so 10 in total if you add 3 people e, f,g you have added now : a-e, a-f, a-g, b-e, b-f, b-g, c-e, c-f, c-g, d-e, d-f, d-g, e-f, e-g, f-g, m-e, m-f, m-g so 18 added and 28 in total  it is also how to select 2 people between n : c(2,n) = n (n-1)/2 = 8*7/2 = 28