We take two imagined classes, one at the beginning, namely (-5)-0, and the other at the end, namely 40-50, each with frequency zero.
With these two classes, we have the following frequency table.
Now, we plot the following points on a graph paper:
A(-2.5, 0), B(2.5,10), C(7.5,16),D(12.5,30), E(17.5,42), F(22.5,50), G(27.5,30), H(32.5,16), I(37.5,12), J(42.5,0).
Join these points by line segments AB, BC, CD, DE, EF, FG, GH, HI, and IJ to obtain the required frequency polygon, shown just above.