There are 10 points in a plane out of which 4 points are collinear. How many straight lines can be drawn by joining any two of them?

From 10 given points, ${ }^{10} C_2$ straight lines can be drawn. But 4 points are collinear, using 4 points, ${ }^4 C_2$ straight lines can be drawn. From 4 collinear points, 1 straight line can be drawn. So, total number of straight lines $={ }^{10} C_2-{ }^4 C_2+1$ $\begin{aligned} & =\frac{10 !}{8 ! 2 !}-\frac{4 !}{2 ! 2 !}+1 \\ & =45-6+1=40 \end{aligned}$