maths

let the pen cost x
then the pencil case cost x+12
buy one pencil case and three pens altogether need 32
x+12+3x=32
4x=20
x=5
a pen cost $5 each

pencil case = $12 + pen
1 pen = pencil case - $12
1 (pencil case) + 3 pen = $32
but since 1 pen = 1 (pencil case) - $12
we substitude pen with pencil case
so
1 (pencil case) + 3 (1(pencil case) - $12) = $32
1 (pencil case) + 3(pencil case) - $36 = $32
4 (pencil case) = $36 + $32
4 (pencil case) = $68
1 (pencil case) = $68/4
= $17
since 1 pen = 1 (pencil case) - $12
so 1 pen = $17 - $12
= $5
Proof:
one pencil case and three pens = $17 + $5 * 3
= $17 + $15
= $32

Hope that helps 8-)

let a pencil case be X & a pen be Y,

X=12+Y...(1)
32=X+3Y...(2)

Sub (1) into (2)
32=X+3Y
32=(12+Y)+3Y
32=12+Y+3Y
32=12+4Y
20=4Y
Y=5

Thus, a pen costs $5 .

Let x be the amoung of money that a pen cost.
(x+12)+3x=32
x+12+3x=32
12+4x=32
4x=32-12
4x=20
x=20/4
x=5
A pen costs $5.

2007-02-03 23:13:26 補充：
Sorry. It has a typing mistake.It should be amount not amoung.Also, the let x statement, it can also be:Let x be the cost of a pen.

$pencil case = $pen + $12

$pencil case + $pen + $pen + $pen = $32
($pen + $12) + $pen + $pen + $pen = $32
4 x $pen + $12 = $32
4 x $pen = $20
$pen = $5

set a pen cost $m

(m+$12)+3m=$32
4m+$12=$32
m=$5

a pen cost $5

let $x be the cost of a pen.
x+(x+12)=32
2x+12 = 32
2x=20
x=10

The cost of a pen is $10