$5.$ For the plentary gear train shown below, gear $2$ has a module of $2$ mm with $75$ teeth; gear $5$ has a module of $4$ mm with $50$ teeth; gear $4$ has $40$ teeth. Determine

$($a$)\backslash (\backslash$mathrm$\{$N$\}3=\backslash )?$

$($b$)$ The speed ratio of the gear train.