6.94%
First, determine how many CDs that Sharon started with by multiplying 240 by 9/2. So
240 * 9/2 = 1080
Now what percent is 75 of 1080? Simple division will give you that.
75 / 1080 = 0.069444444 = 6.9444444%
So Sharon had a 6.94% decrease in the number of CDs she had.