whats happens with 50/50
and say.. with rain prob. = 25% and no rain prob. =75%
I don't know, exactly. But consider this:
Say it rains 1 day out of 160.
If you asked them, every day, whether it was raining, both
of them would lie to you 10 times out of 160. So you've got a 1/160 chance of it raining, and an (approximate) 10/160 chance that they tell you it's raining. If they tell you it's raining, they're very probably lying.
I'm sure there's a way to figure this out in greater detail but that will probably not be on the GMAT.