Q$4.[7$ marks$]$ The present worth of the cash flows through year k$,$ PWk for a defender $(3-$year remaining useful life$)$ and a challenger $(5-$year useful life$)$ are shown in the table below.

Assume the MARR is $\backslash (10\backslash \%\backslash )$ per year. Based on this information,

a$.$ What is the economic life and the related minimum equivalent uniform annual cost, EUAC, for the challenger?

b$.$ Compute marginal cost for defender each year, show your calculations.