<|im_start|>system
You are a helpful assistant. You first think about the reasoning process in the mind and then provide the user with the answer. You should try to truely solve the question by yourself without cheating.  <|im_end|>
<|im_start|>user
 An airline company operates passenger transportation between four cities A, B, C, and D. The prices for some of the direct flights between these cities are as follows: AB is 2000 yuan; AC is 1600 yuan; AD is 2500 yuan; CD is 900 yuan; BC is 1200 yuan. If the company's pricing for flights is directly proportional to the straight-line distance between the cities (assuming all four cities are on the same plane), calculate the price for the straight-line distance between B and D. Express your answer in yuan as a whole number. Show your work in <think> </think> tags. And return the final answer in <answer> </answer> tags.<|im_end|>
<|im_start|>assistant
Let me solve this step by step.
<think>
user
1500
An airline company operates passenger transportation between four cities A, B, C, and D. The prices for some of the direct flights between these cities are as follows: AB is 2000 yuan; AC is 1600 yuan; AD is 2500 yuan; CD is 900 yuan; BC is 1200 yuan. If the company's pricing for flights is directly proportional to the straight-line distance between the cities (assuming all four cities are on the same plane), calculate the price for the straight-line distance between B and D. Express your answer in yuan as a whole number.
test