假设英语课上老师要求同桌之间相互修改作文, 请你修改你同桌写的以下作文。文中共有10处错误，每句中最多有两处错误。错误涉及一个单词的增加、删除或修改。

增加：在缺词处加一个漏字符号（∧）, 并在其下面写出该加的词。

删除：把多余的词用斜线（＼）划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1. 每处错误及其修改均仅限一词；2. 只允许修改10处，多者（从第11处起）不计分。

Dear David,

    Long time no see! I miss you very much.

 I'm writing to express my thank for your help during the seven-day summer camp. I have made a great progress in my English by communicating to you. What is more, while visit some tourist attractions, you volunteer to be my guide and showed me a lot of American culture, it has broadened my horizons. Not only have I learn a lot about American culture, but also I have made good American friends. It was really a wonderful experience!

     By the way, would you like to join the summer camp in China the next year? I'd like to your guide and you can experience any Chinese culture and customs. I am sure you will have a great time.

    I am looking forward to your reply.

Yours sincerely.

Li Hua

横线上面应填几？

____ +5=12     6+____ =15      ____ +8=17

____ +6=12     7+____ =17      ____ +4=12

设m=${\left(2\frac{1}{4}\right)}^{\frac{1}{2}}$﹣（﹣9.6）⁰﹣${\left(3\frac{3}{8}\right)}^{-\frac{2}{3}}$+（1.5）^﹣2；n=log₃$\frac{\sqrt[4]{27}}{3}$+lg25+lg4+$7^{{\log }_{7}2}$．求m+n的值．