The family of giants you work for is throwing a fancy dinner party, and they all want to look their best. But there’s a problem – the elder giant’s favorite  shirt is wrinkled! To fix it, you’ll need to power up… the Giant Iron. The iron needs two giant  batteries to work. You just had 4 working ones and 4 dead ones in separate piles, but it looks like the baby giant  mixed them all up. You need to get the iron working  and press the giant shirt, fast – or you’ll end up being the  main course tonight! How can you test the batteries so that you’re guaranteed to get  a working pair in 7 tries or less? Pause the video now if you want to  figure it out for yourself Answer in 3 Answer in 2 Answer in 1 You could, of course, take  all eight batteries and begin testing the 28  possible combinations. You might get lucky within  the first few tries. But if you don’t, moving the giant  batteries that many times will take way too long. You can’t rely on luck – you need to assume the worst  possibility and plan accordingly. However, you don’t actually need  to test every possible combination. Remember – there are four  good batteries in total, meaning that any pile of six you choose will have at least two  good batteries in it. That doesn’t help you right away, since testing all six batteries could  still take as many as 15 tries. But it does give you  a clue to the solution – dividing the batteries into smaller  subsets narrows down the possible results. So instead of six batteries, let’s take any three. This group has a total of three  possible combinations. Since both batteries have to be  working for the iron to power up, a single failure can’t tell you whether  both batteries are dead, or just one. But if all three combinations fail, then you’ll know this group has either  one good battery, or none at all. Now you can set those three aside and repeat the process for  another three batteries. You might get a match, but if every  combination fails again, you’ll know this set can have  no more than one good battery. That would leave only two  batteries untried. Since there are four good  batteries in total and you’ve only accounted for two so far, both of these remaining ones must be good. Dividing the batteries into  sets of 3, 3, and 2 is guaranteed to get a working result  in 7 tries or less, no matter what order  you test the piles in. With no time to spare,  the iron comes to life, and you manage to get the shirt  flawlessly ironed. The pleased elder and his family show up  to the party dressed to the nines … well, almost. 