6.20 Christmas Trees The diameters of Douglas firs grown at a Christmas tree farm are normally distributed with a mean of 10 cm and a standard devia- tion of 3 cm.

a. What proportion of the trees will have diameters between 8 and 12 cm?

b. What proportion of the trees will have diameters less than 7 cm?

c. Your Christmas tree stand will expand to a diameter of 14 cm. What proportion of the trees will not fit in your Christmas tree stand?