For (a) simply compute the NPW of each project. A project's NPW is given by:
Where is your project and is the variable to denote a period. Since we compute the NPV for periode one and not for zero we have to correct this in the formula. With you denote the interest rate. is the project's cash flow in some period.
You will choose the project with the highest NPV.
For (b) you will get the same result since the NFV is just the . Therefore if then