'elle mange' vs 'elles mangent
I can hear no difference between elle mange and elles mangent; same with masculine pronouns: il mange vs ils mangent. is there an audible difference? Or is this just one of those unsolvable conundrums? (or conundra, if you like 8^) If so, seems like there is ever only a 50/50 chance of getting it right in duolingo!
There is no audible difference between "elle mange" and "elles mangent". Same for "il/ils" In the german duolingo, for "ist/isst", if the context isn't clear, both answers are good. It should be the same for this case. If duolingo says your wrong because you didn't choose the "good" one, send a feedback.
Actually, I've never seen two sentences that both appear on Duolingo and have the exactly same sound. If you remember that you were supposed to write "elles mangent", you won't be mistaken :) This is just a workaround, though. Of course you should send a feedback if you are sure that there is another answer that sounds exactly the same and is grammatically correct.