After-tax real rate of interest and minimum real interest rate??

I need help on these problems like explain it, and have an equation to solve the problem please. Anything would be appreciate it thanks!

- Assume the following;

r = 4%

τ = 0.2

π = 4%

What is the after-tax real rate of interest? ______________

- Assume the following;

r = 4%

τ = 0.2

π = 20%

What is the after-tax real rate of interest? ______________

- Assume a liquidity trap where i = 0% and π = 3%, what is the minimum real interest r? ___________________

- Assume a liquidity trap where i = 0% and π = -8%, what is the minimum real interest r? ___________________

Re: After-tax real rate of interest and minimum real interest rate??

What are i, T, r, and n please specify.