Answer:
Cost of merchandise sold = $483 , Closing stock = $227
Explanation:
Perpetual inventory system includes updates done, when sale or purchase transaction happens
Opening Stock = 26 units (price 15). Value = 26 x 15 = 390
Sale = 13 units, price 15. So, sales cost value =  13 x 15 = 195  
Purchase = 20 units (price 16). Value = 20 x 16 = 320 
Sale = 18 units, price 16. So, sales cost value = 18 x 16 = 288 
Total sales cost value, or cost of merchandise sold = 195 + 288 = 483 
Closing stock = Opening stock + purchase - sales cost 
= 390 + 320 - 483 
= $227