Compute the present value of a $$240$ cash flow for the following combinations of discount rates and times: $($Do not round intermediate calculations. Round your answers to $2$ decimal places.$)$

a$.r=12\%,t=8$ years

b$.r=12\%,t=16$ years

c$.r=6\%,t=8$ years

d$.r=6\%,t=16$ years