Maths

Let x be the greater number, y be the smaller number.

x=y+18 --- (1)
3x-y=78 --- (2)

Put (1) into (2),
3(y+18)-y=78
2y+54=78
2y=24
y=12

Put y=12 into (1),
x=12+18
x=30

Hence the sum of the two numbers: 30+12 = 42

Let the smaller number be a ,the greater number be b.

a = 18 - b -------(1)
3b - a = 78-------(2)

From (1)
a + 18 = b
a = b - 18----(3)

Put (3)into (2)

3b - a = 78
3b - b - 18 = 78
2b - 18 = 78
2b =96
b = 48

Put b = 48 into (1)

a + 18 = b
a + 18 =48
a =30

2008-08-16 13:09:36 補充：
the sum of two number is 48+30 = 78

2008-08-16 13:16:17 補充：
sor. it 's wrong..

a + 18 = b ------(1)
3b - a = 78------(2)

let k be the smaller number
3(k+18)-k =78
2k+54=78
k=12

the sum of the two numbers
=(k+18)+k
=2k+18
=48

smaller number is 48
greater number is 66