Serial sevens is a standard clinical test used by many doctors to assess mental status as part of the mini mental test exam, in cases of confusion, memory loss, and dementia.
Basically you start from 100 and subtract 7 from the answer and then subtract 7 again from that answer. It is a test of concentration and memory, which means you won't see your answer, but must remember it for the next calculation. We doubt you have dementia, so have given you 30 SECONDS to complete the task. A wrong answer will end the quiz.