7 points are marked on a straight line and another 6 points are marked on a second straight line with no points in common. How many triangles can be constructed with vertices from among the above points? Choices.
A) 281  B) 91  C) 168  D) 196  E) 364
Solution: select 2 from 7 and one from 6 + select 2 from 6 and 1 from 7   7C2*6C1 + 6C2*7C1   = 281   281nswer : 281
<result>A</result>
7 points are marked on a straight line and another 6 points are marked on a second straight line with no points in common. How many triangles can be constructed with vertices from among the above points?