SOLUTION: Please help. Following are the speeds of 40 cars clocked by radar on a particular road in a 35-mph zone on a particular afternoon: 30, 36, 42, 36, 30, 52, 36, 34, 36, 33, 30, 32,

I'll choose 6 classes.

Maximum = 52
Minimum = 15

Always round up to the next higher integer, 
never round down.  So we round up to 7

So width of each class is 7.  We start  a unit
before the smallest value or 6.5, then we add 7 and get 
13.5.  So we begin with the first class:

   CLASS
14.5 -- 21.5 

Then we start the next class at the same value the previous
class ended, 21.5, and add 7 again and get 27.5.  So the 
second class is from 21.5 to 27.5 and we write it below the
first class: 

    CLASS
14.5 -- 21.5
21.5 -- 28.5

And we continue the same way through the 6 classes, and we end
up with this

    CLASS
14.5 -- 21.5     
21.5 -- 28.5
28.5 -- 35.5
35.5 -- 42.5
42.5 -- 49.5
49.5 -- 56.5

Now we count the number of numbers in each class, and make a column
of frequencies (the number of numbers in each class).  As a check to
see that you counted correctly, always add the column of frequencies
to make sure you get the the total number of numbers, which is 40

    CLASS        FREQUENCY
14.5 -- 21.5        2
21.5 -- 28.5        4
28.5 -- 35.5       17
35.5 -- 42.5       13
42.5 -- 49.5        2
49.5 -- 56.5        2
---------------------------
       TOTAL       40


To make the histogram, we start off with an x-axis marked
off with the class bounds, and a y-axis, not joined to
the x-axis as in regular algebra, but just sitting off to the
left side marked off with numbers from 0 going up to the highest 
frequency (or more).



Next we draw 6 join rectangles, each one as high as
the frequency of the class whose boundary is at its
bottom left and bottom right corner.




 

That's the histogram.  It is mound shape (or "bell"-shaped) because
it bulges up in the middle.

Edwin