For Problem U-15: (a) $A = I$ (or any example with no eigenvalues equal to 5); (b) $A = \left[\begin{array}{rrr}5&0&0\ 0&1&0\ 0&0&1\end{array}\right]$; (c) $A = \left[\begin{array}{rrr}5&0&0\ 0&5&0\ 0&0&1\end{array}\right]$; (d) $A = 5I$.