F.2 mathz

A two-digit number is given and the sum of its digits is 7.if the tens digit and the units digit are interchanged, then the new number is greater than the original number by 45. find the original number.
Let x be the units digit, y be the tens digit.
x + y = 7............................(1)
10x + y + 45 = 10y + x.......(2)
From (2),
9x + 45 = 9y
x +5 = y..............................(3)
Substitute (3) into (1)
x + x + 5 = 7
2x = 2
x = 1 //
Substitute x = 1 into (1)
y + 1 = 7
y = 6 //
The original number is 16.

2007-03-30 17:48:42 補充：
edmund is 7 years older than his sister. 9 years ago, he was twice as old as his sister. how old is edmund now? Let Edmund be x years old now. His sister be y years old nowx = y + 7.................................(1)x - 9 = 2( y-9 )..........................(2)

2007-03-30 17:49:05 補充：
From (2), x - 9 = 2y - 18x = 2y - 9.................................(3)Substitute (3) into (1)y + 7 = 2y - 9y + 16 = 2yy = 16 //Substitute y = 16 into (1)x = 16 + 7x = 23 //Edmund is 23 years old now. His sister is 16 years old now

let-the(十位數英文)is-x,thx (個位數英文)is-y
{x+y=7---------------------------------(1)
{10x+y=10y+x-45------------------(2)
from(1)
x+y=7
x=7-y-------(3)
(3)sub-into(2)
10(7-y)+y=10y+7-y-45
70-9y=9y-38
70=18y-38
108=18y
y=6
y=6sub-into(1)
x+y=7
x+6=7
x=1
so -x=1,y=6