<|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
 After the start of the school year, a high school randomly selected 90 students from the first-year students for a family situation survey. After some time, they randomly selected 100 students from the same grade for an academic situation survey and found that 20 students had been selected in the previous survey. Estimate the number of first-year students in this school. 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
450
After the start of the school year, a high school randomly selected 90 students from the first-year students for a family situation survey. After some time, they randomly selected 100 students from the same grade for an academic situation survey and found that 20 students had been selected in the previous survey. Estimate the number of first-year students in this school.
test