f2數-solve simultaneous equations

1) let the present's age of a man and his son be x & y,
x-y=22 ---(1)
(x+2)+ (y+2)/2 =55 => 2x+y=104 ---(2)
(1)+(2):
3x=126
x=42, y=20
Therefore, the present's age of a man is 42 yrs old.

2) let the present's age of a man and his son be x & y,
x-y=20 ---(1)
(x+4) = 2(y+4) => x-2y=4 ---(2)
(1)-(2):
y=16, x=36
Therefore, the present's age of a man is 36 yrs old.

3) let the present's age of a man and his son be x & y,
(x-4) = 12(y-4) => x-12y= -44 ---(1)
x+y=47 ---(2)
(2)-(1):
13y=91
y=7, x=40
Therefore, the present's age of his son is 7 yrs old.

2008-02-20 20:46:35 補充：
中文解釋如下1) 列式(1)：爸現在年齡與比仔差22年；列式(2)：2年后，爸年齡加仔年齡一半共55歲。2) 列式(1)：爸現在年齡與比仔差20年；列式(2)：4年后，爸年齡係仔年齡2倍。3) 列式(1)：4年前，爸年齡係仔年齡12倍；列式(2)：爸現在年齡加仔年齡共47歲。