小华用电能表测量家中电热淋浴器的热效率．他将家中其他用电器与电源断开，仅让淋浴器工作，把淋浴器内质量为50kg，初温为20℃的水加热到50℃，加热前后电能表示数如图所示，已知c_水=4.2×10³J/（kg•℃），求：

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请阅读Tony从美国发来的电子邮件，以李华的名义写一篇80词左右的回复。文中不得出现真实的人名、校名与地名。

Hey Li Hua

How are you? I've been busy since I came back from China. Guess what! I'm learning Chinese very hard, but I find it really difficult to learn Chinese well. Please tell me the best way you think.

One more thing, I hear you have Mid-Autumn Festival in China. What's the date of the festival? How do you celebrate?

Tony