The total distance covered from Monday to Wednesday is 12 + 18 + 21 = <<12+18+21=51>>51 miles.

So the average distance traveled per day is 51/3 = <<51/3=17>>17 miles.

# Answer

17
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
If a vehicle is driven 12 miles on Monday, 18 miles on Tuesday, and 21 miles on Wednesday. What is the average distance traveled per day?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
17
If a vehicle is driven 12 miles on Monday, 18 miles on Tuesday, and 21 miles on Wednesday. What is the average distance traveled per day?
train