Let $\mathrm{S} = \{ {w_1},{w_2},......\} $ be the sample space associated to a random experiment. Let $P({w_n}) = {{P({w_{n - 1}})} \over 2},n \ge 2$. Let $A = \{ 2k + 3l:k,l \in N\} $ and $B = \{ {w_n}:n \in A\} $. Then P(B) is equal to :