Four out of five people have had cold pizza for breakfast. If 20 people are asked, what is the probability that exactly 10 have had pizza for breakfast.
"Four people out of five have had cold pizza for breakfast" seems to imply that the probability for success (p)=0.8.
After that, review the conditions for applicability of binomial distribution and apply accordingly. If some of the conditions are not specified, and if you believe that it is appropriate, state your assumptions and proceed accordingly.
For conditions, see:
http://www.jiskha.com/display.cgi?id=1344089015