阅读理解，根据短文内容回答下列问题。
Sam: What do you usually do on the weekend, Mike?
Mike: I usually go to the natureto park to watch insects (昆虫) and collect leaves. Sometimes I take a trip.
Sam: What are you going to do this weekend?
Mike: I am going to read books in the library.
Sam: I like reading very much. Let's go together, OK?
Mike: Great! But the library is far from here.
Sam: Really? How can we get there?
Mike: We can go there by the No.15 bus.
1. What does Mike usually do on the weekend?
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2. What is Mike going to do this weekend?
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3. How will they go to the library?
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，求常数a；
的数学期望．