唔識啊..........

(1) A + B = C
(2) 5A = B + C
(3) A + C = ?

From (1),
A = C - B .....(4)
Put (4) into (2),
5 (C - B) = B + C
5C - 5B = B + C
4C = 6B
C = 1.5B

A + B = C
A = C - B
= 1.5B -B
= 0.5B

A + C = 0.5B + 1.5B = 2B

所以，要放 2 個B (3) 先會平衡。

2007-03-11 12:21:56 補充：
If 5=10^a 和 7=10^b,Then,log 5 = alog 7 = blog (2/7) = log 2 - log 7= log (10/5) - log 7= log 10 - log 5 - log 7= 1 - a - b