This is straight-out an uniform distribution question. The answer is the ratio of the areas of two triangles.
Follow these direction carefully. Draw a line segment \(\displaystyle \overline {AC} \) parallel ti bottom of paper.
The length \(\displaystyle \left\| {\overline {AC} } \right\| = b\). Pick any point \(\displaystyle B\) above the line segment so that we now have \(\displaystyle \Delta ABC\). Draw a dotted line from \(\displaystyle B\) perpendicular to \(\displaystyle \overleftrightarrow {AC}\). That length \(\displaystyle h\) is the height of the triangle. Draw any line to parallel to \(\displaystyle \overleftrightarrow {AC}\) that intersects \(\displaystyle \overline {AB} \) between \(\displaystyle {A~\&~B} \) That line's distance to \(\displaystyle \overleftrightarrow {AC}\) is \(\displaystyle d\). Now we have \(\displaystyle 0<d<h\). There are now two triangles with common vertex \(\displaystyle B\). The ratio of the area of the smaller to the larger is your answer.
