human
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?
assistant
How much time did we spend going to school? ** The total time we spent going to school is 15 minutes + 6 minutes = <<15+6=21>>21 minutes.
How much time do we have left? ** We have 30 minutes - 21 minutes = <<30-21=9>>9 minutes left not to be late.
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