What is the formula to find the number of one on one meetings a group of people
(let's say 10) will need to have to meet with every one else in the group.

I know that if there are 10 people, each of the 10 will need 9 meetings and each meeting will consist of 2 people. Therefore 45 meetings are necessary.

In the reverse, how do I find out how many people are meeting if it takes 66 meetings...
I know the answer is 12 by following the sequences. What is the formula?

HELP. Tks.

You've got two data points already figured out. In both cases you took the number of people (let's call that N), multiplied it by (N-1), and divided by 2 to determine the number of meetings (call that M). So the formula is:

M = N*(N-1)/2, or M = (N^2-N)/2.

Rearrange as a quadratic:

N^2 - N - 2M =0.

Now use the quadratic formula to solve for N.