There are 3 hours * 60 minutes/hour =<<3*60=180>>180 minutes in 3 hours

You can paint 180 minutes / 20 houses/minute = 9 houses in 3 hours.

# Answer

9
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
If it takes 20 minutes to paint a house, how many houses can you paint in 3 hours?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
9
If it takes 20 minutes to paint a house, how many houses can you paint in 3 hours?
train