低form 數學

A比B多25% ........ 以 B 做基數
=> A = B * (1 + 25%) .......... 1 + ?% 因比基數(B)多
=> A = 1.25B ...........

假設B比A少 x%.

B比A少幾多% ........ 以 A 做基數
=> B = A * (1 - x%) .......... 1 - ?% 因比基數(A)少
=> B = 1.25B * (1 - x%)
=> 1 - x% = B / 1.25B = 1/1.25 = 0.8
=> x% = 1 - 0.8 = 0.2 = 20%

所以B比A少 20%.


個答案係20%, 但係我想問應該點計...

2007-10-29 20:41:58 補充：
第一段第三行:=> A = 1.25B ............ (i)第三段第三行:=> B = 1.25B * (1 - x%) ............ from (i)最尾個行係問題, 重複咗, 應該唔要.

set b做x 咁a就係1.25x
之後(1.25x-x)/1.25x = 0.2 (20%)