A restaurant makes $400$ salads per week, each of which contains one$-$half pound of lettuce. The restaurant is open $6$ days per week and typically keeps an inventory of $50$ pounds of lettuce. Which value represents the work$-$in$-$process $($WIP$)?$

$\backslash (1/2\backslash )$ pound of lettuce

$400$ salads per week

$50$ pounds of lettuce

$6$ days per week