數學 Ratio 問題

As 3:20 = 1:(20/3), that means 1 part of bleach needs (20/3) parts of water,
and 200 cm^3 = (1+4) parts, so 1 part = 50 cm^3.
As it needs (20/3 - 4) parts water, ie 8/3 parts water, that is 320/3 cm^3 water.
Ans : the volume of water has to be added is 320/3 cm^3.

係第二個錯都有人投。
第一個只是第二行寫錯，係40，唔係50。其它都冇錯。

A 200cm3 diluted bleach is a mixture of 1 part of bleach and 4 parts of water. Find the volume of water has to be added to dilute the ratio of bleach to water from 1:4 to 3:20.

Let the volume of water has to be added be x cm3
Originally, there is 200*1/5 = 40cm3 bleach and 200*4/5 = 150cm3 water
40 : (150+x) = 3 : 20
40*20/3 = 150 +x
x = 800/3 - 150
x = 350/3
so the volume of water has to be added is 350/3 cm3

2012-11-04 00:22:52 補充：
第一個錯都有人投@@