Supplementary Exercise 9.39 of IPS7e
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Many solutions are possible, and they are all valid (provided the table 
adds up).

With fixed margins, only one of the values a,b,c,d are required to determine the
table. That is, knowing e.g. the value of a, the rest will come automatically.

Some examples:

a=100:  100 100 
        100 100

a=50:    50 150
        150  50

a=200:  200   0
          0 200

a=0:      0 200
        200   0

Clearly, the requirement is that 0<=a<=200.