①通过R₁的电流．

②R₂的阻值．

某班50名学生在一次数学测试中，成绩全部介于50与100之间，将测试结果按如下方式分成五组：第一组50，60），第二组60，70），…，第五组90，100．如图是按上述分组方法得到的频率分布直方图．

（Ⅰ）由频率分布直方图估计50名学生数学成绩的中位数和平均数；

（Ⅱ）从测试成绩在50，60)∪90，100内的所有学生中随机抽取两名同学，设其测试成绩分别为m，n，求事件 “|m-n|＞10”的概率。

    According to a Chinese saying, he who doesn't reach the Great Wall of China is never a great man.

    Growing up in Beijing I1 its historical sites and natural attractions perfectly well. However, I'd 2 climbed the Great Wall before until recently. I, along with my friends3for the top. We soon discovered that the hike was much more 4 than we had imagined. But we didn't 5.It took us about two hours to finally6 where we had wanted to get to.

    As I 7 at the top of the Great Wall, I looked back at the waves of8 walking to the top. At that moment, I saw an amazing 9 that would forever change my opinion on humans' limits. I saw young couples carrying their children, walking10 but surely towards the top. They would stop once in a while to take a11 under the hot sun.

    As I stood there, I began to 12 words that can describe us humans. If one would like to describe Giraffes (长颈鹿), he/she would use words such as 13 and long-necked -But what about us? I took another look down below and my 14began to gather, persevering (坚持不懈的), determined, and 15As these words came into my mind, I 16 that it was not appearance or physical characteristics that should define (定义) us humans, but the 17 that we share.

    I don't know if I became a 18 man after returning from this 19 according to the Chinese saying20 I surely returned as a more thoughtful person.