unit 15

meibaker

hi, i've just started unit 15, but could not understand how the purchases has been calculated, please help.
estimate sales:
march 2
april 3
may 4
june 4
they plan to buy 3 vans in jan, and build up their stock so that by end of Feb, and each following month there willl be enough vans ready for sale to satisfy teh estimate demand of the following two months.
month opening stock purchase sales closing stock
jan 0 3 0 3
feb 3 2 0 5
march 5 4 2 7
april 7 4 3 8