The safest room would be the lions who haven't eaten in three years... Can't have survived that could they?
I don't know about the second one.. I keep changin my mind about whether the hydrogen would ignite the flame or...put it out... Unless the trick is that a person couldn't light a match since they couldn't breathe oxygen...
The greatest weight would be if the flies were sitting on the bottom of the jar, right? They wouldn't be exerting pressure on the scale if they're floating above it...
*shrugs and walks away*