15 minutes is 15/60=<<15/60=0.25>>0.25 hours.

Traveling 15 minutes at 16 miles per hour, Stephen travels 16*0.25 = 4 miles.

Traveling 15 minutes at 12 miles per hour, Stephen travels 12*0.25 = 3 miles.

Traveling 15 minutes at 20 miles per hour, Stephen travels 20*0.25 = 5 miles.

All together, Stephen travels 4+3+5=<<4+3+5=12>>12 miles.

# Answer

12
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Stephen rides his bicycle to church.  During the first third of his trip, he travels at a speed of 16 miles per hour. During the second third of his trip, riding uphill, he travels a speed of 12 miles per hour.  During the last third of his trip, he rides downhill at a speed of 20 miles per hour.  If each third of his trip takes 15 minutes, what is the distance Stephen rides his bicycle to church, in miles?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
12
Stephen rides his bicycle to church.  During the first third of his trip, he travels at a speed of 16 miles per hour. During the second third of his trip, riding uphill, he travels a speed of 12 miles per hour.  During the last third of his trip, he rides downhill at a speed of 20 miles per hour.  If each third of his trip takes 15 minutes, what is the distance Stephen rides his bicycle to church, in miles?
train