It's not going to be a backscatter event if your ping-pong ball hits the frame or these strings, or the top part.

打到原子核了就是背散射,如果打到框上。

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But in our case, the probability of backscattering is going to be the number of balls that backscatter, and that's going to be divided by the total number of ping-pong balls.

用20除以132000，但现在背散射,的概率是被散射回来的小球，除以总的小球数,你们记得。

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