<|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
 A can do a work in 20 days, B in some days. A alone started the work and after 10 days he left the work. B can finish the work in 15 days. In how many days can B do the work? 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
30
A can do a work in 20 days, B in some days. A alone started the work and after 10 days he left the work. B can finish the work in 15 days. In how many days can B do the work?
test