Posted by rere on .
compute the present value of a $100 cash flow for the following combinations of discount rates and times. a. r=8 percent. t= 10 years
b. r=8 percent. t= 20 years
c. r=4 percent. t= 10 years
d. r=4 percent. t= 20 years

finance 
economyst,
an EXCEL spreadsheet is very useful for these types of calculations.
PV = sum[ 100./((1.+r)^i) ]
where i goes from 1 to t.
take it from here.