Question 1049326
The sum of the rates as  {{{BOXES/HOUR}}}  is  {{{200/2+150/3=(200*3+150*2)/6=900/6=450/2=highlight(225(boxes/hour))}}}.


Basic constant rate rule R*h=W to relate rate, time in hours, work.  The unknown time is h for your example.
{{{system(R=225,W=1800*boxes,h=unknownTimeHours)}}}