maths!!

設 x 是該數

x ( 1-5%) (1 + 5%)= 159.6
x (0.95 ) (1.05) =159.6
x (0.9975) =159.6
x =160 //

設該數為y.
(95%y)x105%=159.6
95%y=159.6/105%
95%y=167.58
y=167.58/95%
y=159.201
該數是159.201

2007-11-27 20:17:25 補充：
sorry計錯,部計算機有問題

某數先減少5%,然後增加5%。若最後所得數是159.6,求該數。

設該數為y,

y x(1-5%) x (1+5%)=159.6
y x 95% x 105%=159.6
y x 1.05=159.6 /0.95..............將95%化成0.95,105%化成1.05,調 x 0.95過去變/0.95
y=168/1.05..........調 x 1.05過去變/1.05
y=160..................做完啦>>..

設某數為a

[a x (1 - 5%)] x (1 + 5%) = 159.6
[a x (0.95)] = 159.6 / 1.05
0.95a = 152
a = 160