s1 又一數學難題

The sum of the ages of MrLee and his son is 53. 4 years ago, the age of Mr Lee is4 times that of his son. Find the ages of MrLee's son.


Let f be the age of Mr Lee, and s be the age of Mr Lee's son.

f + s = 53 ...... [1]
f - 4 = 4(s - 4) ...... [2]

From [2] :
f - 4 = 4s - 16
f - 4s = -12 ...... [3]

[1] - [3] :
5s = 65
s = 13

Put s = 13 into [1] :
f + 13 = 53
f = 40

The age of Mr Lee is 40,and the age of his son is 13.

Let y be his son's age now,

4(y-4)+4 =Mr Lee's age now
[4(y-4)+4]+y=53
4y-16+4+y=53
5y-12=53
5y=53+12
y=65/5("/" represents divide by)
y=13

Therefore, his son's age now is 13 (years old).

2011-11-18 22:28:29 補充：
wanszeto is strange that he uses "f" and "s" instead of the common "x" or "y", or even "a" or "b" is acceptable. Another thing is that wanszeto uses 二元一次 instead of 一元一次, which is easier.

不是53.4,是53
4 years ago,the age of Mr Lee is 4 times that of his son(獨立一句)

Let x be Mr Lee's age and y be his son's age
x+y=53.4--------1
x=4y----------2
put 2 into 1
4y+y=53.4
5y=53.4
y=10.68
put y=10.68 into 1
10.68+x=53.4
x=42.72
so Mr Lee's age is 42.72 and his son's age is 10.68


p.s你個數咁怪= =