中二數學，後天7:30A.M.前，20點

27
a)ratio
=2500:4000
=5:8
b)TV=4000*80%=3200$
carmer=2400$
c)
ratio
=2400:3200
=3:4

28
a)ratio of A,B and C
=1*6:3*6:3*5
=2:6:5
b)A get 32500*2/(2+6+5)=5000
B get 32500*6/(2+6+5)=15000
C get 32500*5/(2+6+5)=12500

29
volume of te water
=315*2/(2+7)=70ml
grapes juice's volume=70+25=95ml

30
a)there are
=90/(3/(2+3))=150 students
so there are
=150/(25/(25+2))
=162 teachers
b)
ratio of teachers:students
=160:144
=10:9
students of boys:girs
=(150-90-6):90
=54:90
=3:5

31
let lung's age is yonger than his mother's of 24years
ie. x=y-24...(1), x-3:y-3=1:3...(2)
from (2),x-3=y/3-1
x=y/3+2
then y-24=y/3+2
2y/3=26
y=39
x=15
lung's age is 15 and his mother's age is 39
b)ratio of their age is 15:39
c)15+n:39+n=2:5
75+5n=78+2n
3n=3
n=1after a year later,their age's ratio will be 2:5

now offing...

2011-09-29 08:09:00 補充：
32.
let ratio of xiao(A):yuan(B)=2/3
ratio of Wei(C):B=2:1
ie. B:C=1:2
ratio of A:B:C
=2*1:3*1:3*2
=2:3:6

A get 418*2/11=76
B get 418*3/11=114
C get 228

2011-09-29 08:09:07 補充：
33.
permeater=(18/(1/500)+9.2/(1/500))*2=27000cm=270m
area=(18/(1/500)*9.2/(1/500))=41400000cm^2=4140m^2
4140/6.8 = 608.823529=609 s cor to 3 sig fig

609*2=1218s=20.3mins

zhi wen run about 20.3mins for running a football pitch
(wide=46m,lenght=90m)

好濛，睇唔到呀，介唔介意用文字本打上黎