a box contains 5 white balls and some black
balls. if the probability of drawing a black
ball from the bag is twice the prbability
of drawimg a white ball then find number
of black balls.
It is given a family of open interval set (U_r )_(r∈Q) of R
that satifies condition ∀r∈Q, r∈U_(r ) .
Prove that there exists a family set (U_r )_(r∈Q) which not cover R
or ∀ε>0, λ(∪_(r∈Q) U_r )≤ ε .