Explain Like I'm Five | Don't Panic!

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The original was posted on /r/explainlikeimfive by /u/nikldq on 2023-06-25 09:33:10+00:00.

Original Title: ELI5: Bard has a digital wallet, initial balance is $0. Bard sets it to auto top up $20 if the balance is below $20. Bard always spends $14 for every transaction. What's the max balance of Bard's digital wallet?

This is just a random real-life related question popped up in my mind, not math homework or interview question. I've asked some of my engineer friends but they couldn't give me the general math formula, only the step-by-step iterations.