Five people can mow a lawn in 12 hours.  How many more people are needed to mow the lawn in just 3 hours, assuming each person mows at the same rate?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Five people can mow a lawn in 12 hours.  How many more people are needed to mow the lawn in just 3 hours, assuming each person mows at the same rate?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
15
Five people can mow a lawn in 12 hours.  How many more people are needed to mow the lawn in just 3 hours, assuming each person mows at the same rate?
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