Let. 某數=n
n x (1+15%) x (1-y%) = n (1-31%)
1.15n x (1-y%) = 0.69n
1.15n - 1.15n x y% =0.69n
1.15n - 0.69n= 1.15n x y%
0.46n=1.15n x y%
0.4=y%
y = 40
2007-11-29 17:41:31 補充:
Let. 2001學生人數. = n 2002學生人數. = n x (1+10%) = 1.1 n 2003學生人數. = 1.1 n x(1-5%) 1254= 1.1 n x 0.95 1254=1.045 n n=1200 2001學生人數. = 1200