The average number of sit-ups performed by 15 girls and some boys was 50.
The average number of sit-ups performed by the girls was 80% of the average number of sit-ups performed by all the pupils.
The average number of sit-ups performed by the boys was 30% more than the average number of sit-ups performed by the girls.
(a). What was the average number of sit-ups performed by the girls?
(b). What percentage of the pupils were boys?
Answer:
(a). 50 X 80% = 40
(b). ???
请教高人解答（b), 谢谢。
不知道做的对不对.
The average number of sit-ups performed by the boys was 30% more than the average number of sit-ups performed by the girls.
so average number of st-ups is 30%*40 + 40=52
if the average of 15 girls change from 50 to 40, means the total number of sit-ups boys performed is more than that of girls (50-40)*15=150, one boy is only more than 2 sit-ups, so 150/(52-50)=75, the total number of boy is 75
75/(75+15)*%=83%
40%
不是吧,这就是个简单的代数题!!就是个x,y嘛。六年级题和国内4,5年级的一样。
原题：The average number of sit-ups performed by 15 girls and some boys was 50.
The average number of sit-ups performed by the girls was 80% of the average number of sit-ups performed by all the pupils.
The average number of sit-ups performed by the boys was 30% more than the average number of sit-ups performed by the girls.
(a). What was the average number of sit-ups performed by the girls?
(b). What percentage of the pupils were boys?
英文红色部分是解答，中文即括号里的部分辅助说明一下 ＝）
a) 50 X 80% = 40 sit-ups (平均每个女生)
b) The average number of sit-ups performed by the boys was 30% more than the average number of sit-ups performed by the girls.
100%+30%=130% (the average number of sit-ups performed by the boys was 130% of that performed by the girls)
40×130％＝52 sit-ups (平均每个男生)
52-50=2 sit-ups (平均每个男生比总平均多做两个)
50-40=10 sit-ups (平均每个女生比总平均少做十个)
（换言之，每个女生少于总平均的部分需要五个男生来持平）
可以直接写 No. of boys : No. of girls = 10 : 2 = 5: 1 (第一，男生女生数量比例和男生女生同总平均值差出的量成反比； 第二，提醒孩子化比例为最简) What percentage of the pupils were boys?
5 ／ （5+1）x 100% = 83 1/3 % (由于电脑不能输入除号，用 ‘/’ 代替，家长解释的时候提醒孩子）
同时 数学答案，即使是关于百分比的，也不可四舍五入，有需要的情况下，需用分数表示，如此题)
很多学校不要求写单位 但是个人建议孩子养成好习惯 因为将来判卷老师不一定不介意孩子忽略单位 或者缺少步骤 没必要让孩子冒险吃亏
写出每一个步骤 说明孩子条例清晰 王家长解释时 提醒孩子
＝）