數學的問題!!!!!!!!!!!!!!!!!!!!!!!!!!

let she buy x $2 stamps
and y $5 stamps

2x+5y=54---------(1)
x+y=15------------(2)

(2)x2 2x+2y=30--------(3)

(1)-(3) 2x+5y-2x-2y=54-30
3y=24
y=8

for y=8
x+y=15
x+8=15
x=7

solet she buy 7 $2 stamps

You need to buy 8 $5 stamps and 7 $2 stamps.
你應該買8個5元郵票和7個2元郵票。
因為8×5+2×7=54

設所有的是5元：15×5=75，
要把5元換2元的差是3元，
把75元減54元=21元，
把21÷3(即是差，把5元換2元)=7，
把15-7=8，
所以是有8個5元和7個2元，
即8個5元郵票和7個2元郵票。

這是雞兔問題！！

Let the number of $2stamps she buys be x
5(15-x)+2x=54
75-5x+2x=54
75-3x=54
-3x=54-75
-3x=-21
x=7
.
. . the number of $2tamp she buys is7

Let the number of $2 stamps be X
the number of $5 stamps be (15-X)

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

The number of $2 stamps is 7
and the number of $5 stamps is (15-7)=8

seven $2 stamps .Because $5 x 8 =$40. It say 15 stamps.That now have 7 stamps.And than the $2 x 7 = 14.So $40 + $14 =$54.And also it say 15 stamps.So we need to 8+7=15 . Is that OK.

Let Donna buys y $2 stamps and z $5 stamps
y+z=15---------------(1)
2y+5z=54-----------(2)

From (1),
z=15-y---------------(3)

Substituting (3) into (2),
2y+5(15-y)=54
2y+75-5y=54
21-3y=0
3y=21
y=7
∴Donna buys 7 $2 stamps