maths problem

2007-10-08 2:51 am
solve the problem by the method of elimination

1}ken spent $720 to buy a total of 12CDs and VCDs.If the prices of each CD and each VCD are $80 and $50 respectively,how many CDsand VCDs did ken buy?

2}there are 950 students in a secondary school.if the number of boys is 32more than that of the girls,how many boys and girls are there in the school?


PS.i need steps !!!THANK YOU
更新1:

the value of two digit number is3 times the sum of the two digit.if 9 is added to this two digit number,the new value will be 4 times the sum of the two digit.find the two digit number PS.i need steps !!!THANK YOU

回答 (1)

2007-10-08 3:11 am
✔ 最佳答案
1)
let the numbers of cds be x,vcds be y

80x+50y=720----1
x+y=12----2

from1
8x+5y=72----3
from2
5(x+y)=(12)5
5x+5y=60----4

3-4
8x+5y-(5x+5y)=72-60
3x=12
x=4

sub x=4 into 2

x+y=12
4+y=12
y=8

therefore,he can buy 4 cds and 8 vcds.

2)
let the numbers of boys be x,girls be y

x+y=950---1
x-y=32----2

1+2
x+y+(x-y)=950+32
2x=982
x=491

sub x=491 in to 1

x+y=950
491+y=950
y=359

therefore there are 491 boys and 359 girls in this school.

2007-10-08 22:02:53 補充:
呢度唔夠位寫,俾e-mail我,我send俾你

2007-10-09 21:40:25 補充:
我計左出黎係十位係2,個位係7..成個數即係27..我代過係岩既,但係我LET左3個UNKNOWN,有D寄怪...係咪超趕?俾D時間我研究下先

2007-10-11 22:21:02 補充:
let the ten digit be x , unit digit be y3(x+y)=10x+y--------14(x+y)=10x+y+9---------2約間:2y=7x------1 3y=6x+9-----21)X36y=21x--------32)X26y=12x+18--------4

2007-10-11 22:21:27 補充:
3-46y-6y=21x-(12x+18)18=9xx=2sub x=2 into 36y=21(2)6y=42y=7therefore,the ten digit is 2 , unit digit is 7 and the whole number is 27
參考: myself


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