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?
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
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)
40x130%=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)