In one residence, cell phones, lap top computers and digital tvs are very popular among students. In fact, all of the students own atleast one of these items, although onlu 15 own all 3. Cell phones are the most popular with twice as many students owning cell phones as own lap tops. and digital tvs are still rare, since only half as many students own these as own laptops. As it happens all students who own a digital laptop also have a cellphone.
If there are 10 students who own both a cellphone and a digital tv, but not a laptop, and there are 20 students who don't own a cellphone, how many student live in the residence?