5. I wonder the weight of this letter.
我想知道这封信有多重。
还能这样说：
I want to know how heavy this letter is.
I wonder how much this letter weights.
谚语：
A wonder lasts but nine days.
什么事儿都新鲜不了多久。
6. How long do you think it'll take to get to Beijing?
多久才能到北京？
还能这样说：
Do you think when it will arrive in Beijing?
How long will it spend getting to Beijing?
谚语：
The wise man has long ears and a short tongue.
聪明人听得多说得少。
7. Is there anything valuable in it?
里面有什么贵重物品吗？
还能这样说：
Are there any dear goods in the envelope?
May I ask if there is anything valuable in it?
应用解析：
good value 花得值，值得买；
of no value 没有价值的；
set at no value 认为没有价值；毫不重视；
value oneself on 以……自豪；以……夸耀自己
8. A registered letter must enclose the complete address of the sender.
挂号信必须提供寄件人的详细地址。
还能这样说：
The sender's name and address of registered mail should be written in detail.
You are asked to enclose your detail address by registering letter.
应用解析：
send sth. in 将某物寄至某处进行处理；
send sth. off 寄出或发出某物；
send sb. to 使某人进入某处或某机构