利息嘅問題

1. 設年利率為r%
1000 * r% * 3 = 540
r% = 540 / (1000 * 3) = 0.18
r/100 = 0.18, r = 100 * 0.18 = 18
年利率是18%, 本利和是$1000 + $540 = $1540

2. 設本金為$x
x * 4% * 2 = 160
x = 160 / (4% * 2) = 160 / 0.08 = 2000
本金為$2000, 本利和是$2000 + $160 = $2160

3. 設時期為 t 年
3000 * (1 + 6% * t) = 3900
6% * t = 3900 / 3000 - 1 = 0.3
t = 0.3 / 6% = 0.3 / 0.06 = 5
時期為5年, 單利息為$3900 - $3000 = $900

4. 設原本月薪為$x
x (1 + 25%) = 12600
1.25 * x = 12600
x = 12600 / 1.25 = 10080
原本的月薪是$10080