triangles and lines There are 12 points in a plane. If 4 of them are on a straight line and no other 3 points are on a straight line, then find the difference between the number of triangles and the number of straight lines that can be formed using these points.
A. 215  B. 216  C. 156  D. 155

Hint : Find the number of cases when no $3$ points are collinear. Then subtract the wrong cases.
Number of triangles : $ ^{12}C_3-{} ^4C_3$ // simply subtract all cases when you counted all 3 collinear points  
Number of lines : $^{12}C_2-{}^4C_2+1$ // you have subtracted the cases where you took the 2 points of in same line but that is also a line. You again add 1. 
