4月:sales ;                 100,000                         5月:sales :               80,000
 
less; cost of gd sold:                                            
opening stock : 70,000                                      opening stock : 60,000purchase      ; 20,000                                       purchases    : 15,000
closing stoc  : 60,000    30,000                             closing stock: 70,000   5,000
            gross profit  70,000                               gross profit         75,000唔明Gross profit (5月)係會多過4月