In the first question ,
the previous class is easy to plot as 0-100 and then the mid point 50 can be used, so really no need to draw the kink.Supposing your class intervals start from 40-50, then next one 50-60, 60-70 etc.For the frequency polygon all you need is the mid point of the previous class.
So , that will be the mid point of 20-30 i.e. 25.
So you can plot the mid point of the previous class and then from the origin to that point you can show the kink. Kink is basically a break on the axis.
In the second case ,select a class with minimum class width, say class 1-2
consider that when the width of the class is 1(for the class 1-2), then the frequency is 5,
Now we will modify the lengths of the rectangles to be drawn, to be proportionate to the class size 1.
So, we will think like this,
when the class size is 1, the length of the rectangle is 5
so when the class size is 2(as in the case of class 3-5), the length of the rectangle must be
(6/2)*1=3 , where 6 is the frequency of that class,
So, basically to find the length of each rectangle of the histogram,
you must calculate
(frequecy/class width of the class)*1 as we have taken the lengths of the rectangles should be modified proportionate to class size1.