幫忙一算英文數學題...10分(方程式)

(1)
Let the number of $2 stamps he buy be y,
then the number of $5 stamps he buys is (15-y)
2y+5(15-y)=54
2y+75-5y=54
3y=21
y=7
Therefore he buys 7 $2 stamps.

(2)
Let the elder sister receive $y,
then the youger sister receives $(120-y)
y/5-(120-y)/6=13
6y-5(120-y)=390
6y-600+5y=390
11y=990
y=90
Therefore the elder sister receives $90, then the youger sister receives $(120-90)=$30.

1.
Let x be the number of $2 stamp that Donna buys
Let y be the number of $5 stamp that Donna buys

x + y =15 ............................1
2x+5y=54 .............................2

1x2 = 2x+2y=30.................3
2-3= 3y=24
y=8
because x+y=15
so x+8=15
x=7
so she buy 7 $2 stamp

2.
Let x be the elder sister received
Let y be the younger sister received

x + y=120
x=120-y........................1
x/5-y/6=13
x-5y/6=65
6x-5y=390.......................2

6(120-y)-5y=390
720-6y-5y=390
330=11y
y=30

because
x+y=120
x+30=120
x=90

so the elder sister received $90
the younger sister received $30

1. Let x be the number of $ 2 stamp and the number of $5 stamp 15 -x .

2 x + 5 ( 1 5 - x ) = 5 4
2 x + 7 5 - 5 x = 5 4
- 3 x = - 2 1
x = 7

She buy 7 $ 2 stamp.

2 Let x be the money elder sister has.
and younger sister has 120 -x .

1 / 5 x - 1 / 6 ( 120 -x ) = 1 3
1 / 5 x - 2 0 + 1 / 6 x = 1 3
1 1 / 3 0 x = 3 3
x = 9 0


Elder sister has $90 and younger sister has $30

1. let there are x $2 stamp
so no. of $5 stamp = 15-x

2x + 5(15-x) = 54
2x + 75 -5x = 54
3x = 21
x = 7

so she buy 7 $2 stamp


2. let elder sister receive $x, so younger sister = $(120-x)

(1/5)x - (1/6)(120-x) = 13
6x - (600-5x) = 390
11x = 990
x = 90

so elder sister receive $90
younger sister = $(120-90) = $30

1.let x be the $2 stamps she has
y be the $5 stamps she has
x+y=15 --------- 1
2x+5y=54 ----------2

Form 1,
x=15-y ------- 3

Sub 3 into 2,
2x+5y=54
2(15-y)+5y=54
30-2y+5y=54
3y=24
y=8

so, she buy 8 $2 stamps

2. let x be the money the elder sister received
y be the money the younger sister received
x+y=120 ------- 1
(1/5)x-(1/6)y=13 --------- 2

Form 1,
x=120-y ------- 3

Sub 3 into 2,
(1/5)x-(1/6)y=13
(1/5)(120-y)-(1/6)y=13
(120-y)/5-y/6=13
720-6y-5y=390
-11y=-330
y=30

So, the elder sister received $120-30=$90
the younger sister received $30

2006-10-31 19:13:43 補充：
[修正] so, she buy (15-8)=7 $2 stamps