Of 320 consumers polled, some purchase Blu-ray movies, some only purchase movies on DVD, and some do both. If 300 people polled purchase Blu-ray discs and 280 purchase DVDs and Blu-ray discs, how many people only buy DVDs?

Answer 1

Since B+D-(B&D) = 320,

300 + D - 280 = 320
D = 300

300-280 = 20 buy only DVDs.

Or, you can reason that if 300 bought Blu-ray, and 280 of those also bought DVD, then only 20 bought just Blu-ray.

So, That means that the remaining 20 bought only DVDs