I feel your pain. My stepson did the exact same thing at that age and it went on for years - asked questions all the time that we could not or did not really have to answer, things he was perfectly capable figuring out for himself. We did the same thing, just asked him, "what do you think?". Honestly, it was just a phase that he eventually matured out of but drove us nuts in the meanwhile. Once I said something to him about all the silly questions he kept bugging us with and his response was that he was a kid and he was supposed to ask questions. *Sigh*
He also went through a phase where he was obsessed with time - he needed to know what time dinner would be or what time we would reach our destination if we were driving somewhere, or how many minutes it would take. Or he would ask what time something would happen, we would tell him, and then he would ask, "So that's in X minutes?". We had to start telling him approximate times, and sometimes just saying, "I don't know." or "We'll get there when we get there."
The other thing was he would ask something, we would give an answer, and then he would say, "Oh really?" or "It is?" so we ended up answering more than once. "YES!"
Personally I think it was all about attention - he has a brother a year older that was never like that and he probably did these things to get more attention focused on him. He's finally outgrown it (thank God!). Sorry I don't have any great solutions but maybe trying to give her attention at other times will help. And keep asking her to figure it out for herself. Or ask her why she needs to know. Or say "Just because." or "No reason." Or ignoring it. Or telling her she has a 3 question limit per day. Or you are just not going to answer any questions that she should already know the answer to.
EDITED TO ADD: Read the other responses - love Riley J.'s approach!