If the 15 points are labeled A,B,C,D,E,F,G,H,I,J,K,L,M,N,O then since two points determine a line, and since order doesn't matter (since AB and BA determine the same line), the answer is "15 things choose 2" which is called "the number of combinations of 15 things taken 2 at a time", which is sometimes written 15C2, or C(15,2) or 15C2 or. It is calculated as Edwin