mth 簡單一問

呢類題目要小心某件物件的排位,唔係就好易計錯.

A is 20% larger than B
=> A = B(1+20%) ---(1)

B is 20% smaller than C
=> B = C(1-20%) ---(2)

sub (2) into (1),
A = [C(1-20%)](1+20%)
A = 0.96C
A = C(1-4%)

Therefore, A is 4% smaller than C.

P.S.
A比C小4% 同 C比A大4% 是不一樣的.
前者為: 　　A = C(1-4%) = 0.96C
後者為:　 　C = A(1+4%)
　　　　=> A = C / (1+4%)
　　　　=> A = C (0.9615...) =/= 0.96C

另外,記住文中的排列,就方便寫式架啦.
"X=Y (1 + n%)" if X is n% larger than Y.
"X=Y (1 - n%)" if X is n% smaller than Y.

A is 20% larger than B ----> A =120% B
B is 20% smaller than C---> C = (1-20%)B

Let A=1
A =120% B
B = 5/6
C =(1-20%)B
C =(1-20%)(5/6)
C=2/3
A:C= 1 : 2/3
∴A is 33.3% larger than C