A survey of 400 farmers showed that of the farmers, 122 grew only wheat, 108 grew only corn, 87 grew only oats, 197 grew wheat, 63 grew wheat and corn, 50 grew wheat and oats, and 178 grew com Determine the number of farmers who a) grew at least one of the three, b) grew all three, c) did not grow any of the three, and d) grew exactly two of the three.