數學求救..(financial)

calculate the principal that must be invested now

to provide:a) $2500 in 5 years when the rate is 6又3/4% compounded annually.b) $525 in 4 years when the rate is 5又1/2% compounded quarterly.我需要作法 .. 和答案 ..最好是解釋給我聽也可以 ~因為是ISU .. 所以我很無助 = . = ..
a) $2500 in 5 years when the rate is 6又3/4% compounded annually.2500*(1 6.75%)^5=3465.61b) $525 in 4 years when the rate is 5又1/2% compounded quarterly525*(1 5.5%)^4=650.38
計算本利和 = 本金