The famous batteries and flashlight logic puzzle

[Retro]

The solution is really clever. Can you figure it out before watching the video?

This is a famous problem that has been used in technical interviews, and it has even appeared on a Mathematical Olympiad. There are 4 good batteries and 4 bad batteries, and you need to insert 2 good batteries to make a flashlight work. How many tests do you need?

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[Crims]

Manually working out - the combinations would be 8-25. It's probably the sum of all combined /2 and then minus that 8-25 number.