Let `C` is the set of people who watch Cricket, `F` is the set of people who watch Football, `T` is the set of people who watch Tennis.
Then,
`n(CuuFuuT) = n(C)+n(F)+n(T)-n(CnnF)-n(FnnT)-n(CnnT)+n(CnnFnnT)`
As, `50` people do not watch any game,
So, total number of people who watch any of the games `= 500 - 50 = 450`
`:. n(CuuFuuT) = 450`
`:. 450 = 285+195+115-45-50-70+n(CnnFnnT)`
`=>n(CnnFnnT) = 450-430`
`=>n(CnnFnnT) = = 20`
So, there are `20` viewers who watch all the games.