There were 18 technical questions administered by a very friendly HR. Each question involved something about Euclidean geometry, basic probability theory (usually expectation of some quantity), numerical approximation, or a Fermi problem. The time for each question ranged from 20 seconds for the easier ones to 90 seconds for the more challenging ones. I'd estimate you need to solve around 10 of the questions to pass. The HR stops the clock if you are confused about the question or need clarification.