Answer : A line is formed by joining two points.
If the total number of points is 18, the total number of lines would be = 18C2
But 5 points are collinear, so the lines made by these points are the same and would be only 1.
Hence there is 1 common line joining the 5 collinear points.
As these 5 points are also included in 18 points so these must be subtracted from the total case, i.e. 5C2 must be subtracted from 18C2 .
Finally, the number of straight line = 18C2 – 5C2 + 1
= 144 lines