opacity of overlapping partially transparent elements

Question

if you have one <div> element with an opacity of .5 and another <div> element overlaying the first with an equal opacity then what is the opacity of the two put together?
It's not,

• .5 × .5

nor is it,

• .5 + .5

How do I calculate such a problem?

Answer 1

I believe the formula you are looking for would be 1-(x × y)