When a girls says "I'll let you know" what does it mean?

Anonymous

If you ask a girl to go do something with you, like going to a sports game or bowling or whatever, and she says "I'll let you know" is she just letting you down gently? I told this girl to let me know in two days and she repeated "two days" but from her tone and body language, I'm not sure if she's seriously considering it.

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+1 y
Actually, guys, it turned out that she eventually said "yes" in two days. Maybe she was just old-fashioned and needed to check with her parents or something. Or maybe she really had to think about it.
When a girls says "I'll let you know" what does it mean?
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