數學問題(幫忙)(請用完整的步驟)

Let the number of girls be x,
x + x-11=45
x=28

There are 28 girls, 17boys

Let number of $2 stamp be x, $5 stamp be y
2x +5y =54 (1)
x+y=15 (2)

put (2) into (1)

2(15-y) + 5y=54
3y= 24
y=8

so she buys 8 $5 stamps and 7 $2 stamps

First, presume girl is x, then, boy is 45-x
from the question
x-(45-x)=11(the number of girl is more than the numbers of boys by 11)
x-45+x=11
2x=56
x=28
hence, girl is 28 and boy is 17

Second, presume $2 stamp is x, then, $5 is y
x+y=15(..gets 15 stamps.. )(1)
2x+5y=54(..she spends $54 altogether ..)(2)
from (1)
y=15-x(join to(2))
2x+5(15-x)=54
2x+75-5x=54
-3x=-21
x=7(join to (1))
7+y=15
y=8
hence,May buys 7 $2 and 8 $5 stamps

Let the number of boy is X
X+X+11=45
2X=34
X=17

SO the number of girl is 28 the numbers of boys is 17

Let May buy $2 stamps x,May buy $5 stamps y
2x+5y=54...1
x+y=15...2

2--y=15-x..3

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

7+y=15
y=8

SO she buy $2 stamps 7, she buy $5 stamps 8

let x be the student of boys,
2x+11=45
2x=34
x=17
there are 17 boys in the class,
there are 17+11=28 girls in the class.

let y be the number of $5 stamp,
2*(15-y)+5y=54
30-2y+5y=54
3y=24
y=8
May buys 8 $5 stamps & 15-8=7 $2 stamps

2006-10-17 18:33:38 補充：
慢左=.=