It's a 30% chance. If say you had to pick a number between 1-20 to get a legendary out of the engram, roughly 14 of those numbers would be wrong, and only 6 would be right. That's the same for your 1st one, and the 50th. A new set of 14 losing numbers, and a new 6 winning.
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furthermore. If I pickup a "rare" engram 100% of the time I get a rare item. So why is a "legendary" engram 30% chance? awful mapping. Should have listened to the thousands of reviews out there and not bothered with this game!