The numbers of straight lines can be formed out of 10 points of which 7 are collinear?

If there were no three points collinear. We should have ¹⁰C₂ lines but since 7 points are collinear we must subtract ⁷C₂ lines and add the one corresponding to the line of collinearity of the seven points.
Thus, the required number of straight lines .
= ¹⁰C₂ - ⁷C₂ + 1 = 25