<|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
 Five identical balls are moving in one direction along a straight line, and five other identical balls are moving in the opposite direction. The speeds of all the balls are the same. When any two balls collide, they rebound in opposite directions with the same speed they had before the collision. How many collisions will occur in total between the balls? 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
25
Five identical balls are moving in one direction along a straight line, and five other identical balls are moving in the opposite direction. The speeds of all the balls are the same. When any two balls collide, they rebound in opposite directions with the same speed they had before the collision. How many collisions will occur in total between the balls?
test