Teamfight Tactics (TFT) – How to Calculate Interest?
Assuming the challenge only counts interest earned from gold you already have, here are the mechanics. Basically, you collect interest for every 10 gold you have, up to 50. Interest is calculated at the end of every round and provides +1 gold for every 10 gold you have. So if you have 10 gold, you get one gold in interest. If you have 20 gold, you get to gold in interest, and so on. Having 50 gold will get you the maximum five gold interest.
Now the challenge calls for 20 gold in interest in one stage, which is usually comprised of four to five rounds (2-1, 2-2, 2-3, and so on). That means your best bet in accomplishing this is to save up gold and hope to have around 50 by mid-game. Completing four rounds in stage three, for example, would give you the necessary 20 gold in interest that you need.
Unfortunately, this also means you’ll have to be saving up gold for the game, which means no re-rolling and limited leveling up. This isn’t necessarily a bad thing, though, as most high-level players recommend hoarding gold until late game anyway.
To sum up the challenge “Mine 20 gold in one stage,” simply save your gold for a game.
Why Riot has such confusing wording on it is beyond me, but hopefully it’s fixed in the future. It should be noted that there are also reports that this challenge is bugged (at the time of this writing), so cross your fingers that it works.