$4\mathrm{.}78$ Use Eqs. $(4\mathrm{.}5)$ and $(4\mathrm{.}6)$ to determine the moment of the $20-N$ force about $($a$)$ the $x$ axis, $($b$)$ the $y$ axis, $($c$)$ the zaxis. $($First see if you can write down the results without using the equations.$)$