posted by Berta .
of 320 consumers polled, some purchase Blu-ray movies, some purchase movies on DVD, and some do both. If 300 people polled purchase Blu-ray discs and 280 purchase DVD's and Blu-ray discs, how many people only buy DVD's?
300+280-320 = 260
So, 260 bought both.
That means that 280-260=20 buy only DVDs