Imagine a question and stream-of-consciousness explanation for which this is the answer: 9
Question: My brother and I have thirty minutes to go to school, without being late. It takes us 15 minutes to arrive at the school gate, and another 6 minutes to get to the school building. How much time do we have to get to our room, without being late?
Stream-of-consciousness: The total time we spent going to school is 15 minutes + 6 minutes = 21 minutes. We have 30 minutes - 21 minutes = 9 minutes left not to be late.