Maths!!!

let A,B and C be the weight of the rice and A:B:C= 1/5:1/4:1/3
A+B+C=94
then we need to express B and C in term of A
A:B= 1/5: 1/4
A/B= (1/5)/(1/4)
A/B= 4/5
B=5A/4

A:C= 1/5:1/3
A/C=(1/5)/(1/3)
A/C=3/5
C=5A/3

A+B+C=94
A + 5A/4 + 5A/3 =94
12A/12 + 15A/12 + 20A/12=94
(12+15+20)A/12=94
47A/12=94
A=24

so B=5A/4=5*24/4=30
and C=5A/3=5*24/3=40

the weight of them are 24kg, 30kg and 40kg

1/5 : 1/4 : 1/3 = 12/60 : 15/60 : 20/60
= 12:15:20

Let C be the constraint so that C(12+15+20) = 94

47C=94
C=2

Thus the weight of these 3 portions is 24, 30, 40 respectively

2008-06-25 23:21:25 補充：
對呢McKing，rewrite the ratio才是最佳的approach!

現在才答, 勝算不大了~
但本人有一個方法，或者比較好。
3個ratio 係1/5 :1/4 :1/3 = 12 : 15 : 20
第一舊的重量: 94*(12/47)
二: 94*(15/47)
三: 94*(20/47)