數學..............

首先,
(5cm*5cm*pi)*4cm咁大o既圓柱體要$30,
即係(100*pi)cm^3咁大o既圓柱體要$30,
1cm^3就要 $30/(100*pi) 咁多錢.

r=4cm,h=6cm時餅的價格:
(4cm*4cm*pi)*6cm * [$30/(100*pi)]
= $28.8.

明唔明?

pi x (5)^2 x 4 =100pi
pi x (6)^2 x 4 =144pi
30 / 100pi x 144pi = $36

2007-08-25 16:53:29 補充：
pi x (4)^2 x 6 = 96pi30 / 100pi x 96pi = $28.8sor~

pi x r^2 x h = price x k, where k is the constant
pi (5^2) (4) = 30k
k = 100pi / 30
= 10/3 pi

pi x r^2 x h = price x k
pi (4^2) (6) = price (10/3 pi)
price = 28.8