Let the set X={x1, x2, x3}, if P({x1})=.4 and P({x1,x3})= .5 find probability of P(power set of X) =? ....P= probability.

Attempt. Since probability of a set is the membership function m(x1) for instance, then I thought that probability of the power set will the Sum of probability of all of the sets.

That is P(power set X)= .4+.5+ bunch of 0= .9.

But I am really not sure. if anyone can help it wil be so awesome! I am just confused by this. I'll have a fuzzy set theory like problems to do.