(三)數學唔識做(快...)

1.2x/5 - x/10 = 11/20
4x/10-x/10=11/20
3x/10=11/20
x=11/20×10/3
X=11/6
2.比賽有660人，佔全校人 82.5(%) ，此校有多少人?
660÷82.5%
=660×100/82.5
=800

3.汽水半打18元，賠率25%，求每支的成本 賠率??
18÷6÷75%
=4

4.姨的歲是妹的5倍，若姨比妹大36歲，妹是?
36÷(5-1)
=9

5.computer 用85折 出售，便宜了450 元 ，原價(computer) 是?
450÷15%
=3000

2007-03-31 19:11:15 補充：
順便講埋用方程計4 設妹妹的歲數是x歲則姨的歲數是5x5x-x=364x=39x=92. 設此校有x人82.5%x=660x=660÷82.5%x=8003設每支的成本是x6×75%x=1875%x=3x=45 設原價(computer) 是x15%x=450x=450÷15%x=3000

let x be the total number of student
x(0.825)=660
x=660/(0.825)
x=800
there are 800 students at school

let x be the orininal price of coke
x(1-0.25)=18
x=24
the ofininal price of coke is 24

let x be the age of sister
let y be the age of aunt
5x=y.............1
y-x=36...........2
subsitite 1 into 2
5x-x=36
4x=36
x=9
ut x=9 int o1
5(9)=y
y=45
the age of sister is 9 and the age of aunt is 36


let x be the original price of computer
x(1-0.85)=450
x=450/0.15
x=3000

1.11/20
2.800
3.不知道
4.不知道
5.3000

姨的歲是妹的5倍，若姨比妹大36歲，妹是7.2
歲