http://www.businessinsider.com/the-hardest-apple-interview-questions-2017-5/#we-have-a-cup-of-hot-coffee-and-a-small-cold-milk-out-of-the-fridge-the-room-temperature-is-in-between-these-two-when-should-we-add-milk-to-coffee-to-get-the-coolest-combination-earliest-at-the-beginning-in-the-middle-or-at-the-end-product-design-engineer-candidate-17