If the probability distribution of a discrete random variable $X$ is given by $P(X=k)=\frac{2^{-k}(3 k+1)}{2^c}, k=0,1,2, \ldots \ldots \infty$, then $P(X \leq c)=$

The probability that a person $A$ completes a work in a given time is $\frac{2}{3}$ and the probability that another person $B$ completes the same work in the same time is $\frac{3}{4}$. If both $A$ and $B$ start doing this work at the same time, then the probability that the work is completed in the given time is