<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?
<bot>: 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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