Out of 18 points in a plane, no three are in the same straight line except five points which are collinear. How many straight lines can be formed by joining them ?
Answers:
A) 124
B) 144
C) 134
D) 142
E) 143
The number of straight lines formed joining the 18 points, taking 2 at a time = 18C2=153. Number of straight lines formed by joining the 5 points, taking 2 at a time =5C2=10. But 5 collinear point, when joined pairwise give only one line. Required number of straight lines =153-10+1= 
<gadget id="calculator">153-10+1</gadget>
<output>144</output>
144. Answer :144
<result>B</result>
Out of 18 points in a plane, no three are in the same straight line except five points which are collinear. How many straight lines can be formed by joining them ?