First convert 90 minutes to hours: 90 minutes / 60 minutes/hours = <<90/60=1.5>>1.5 hours.

In 1 hour, I run 12 miles / 1.5 hours = <<12/1.5=8>>8 miles/hour.

# Answer

8
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
I run 12 miles in 90 minutes. What is my average speed in miles per hour?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
8
I run 12 miles in 90 minutes. What is my average speed in miles per hour?
train