數學]]好好急急急急.........please有20點呀

不知是否這樣理解
Find simple interest amount
1.) $500 2% (p.a.) 2 (years) = 500x 2% x 2= $20

Find Interest rate p.a.
2.) $1000 3 (year) $540 (after 3 years interest) = 1000 x ?% x 3 = 540
?% = 540 / 1000 / 3
?% = 0.18
? = 0.18x100
?=18%

Find principal
3.) 4% (p.a.) 2 (years) $160 (after 2 years interest) = ? x 4% x 2 = 160
? = 160 / 4% / 2
? = 2000

Find period
4.) $3000 6%(p.a.) $3900 (after ? years interest + principal)
= 3000 x 6% x ?years = 3900 - 3000
180 x ? = 900
? = 5years

希望幫到你啦^^