Answer:
You would have 16 mattresses available for sale after the delivery on Friday morning.
Step-by-step explanation:
Because, on Friday morning, you received a new order of 15 mattresses, which would put the total number of mattresses in the warehouse at 21. But, 7 mattresses were sold on Monday, 2 mattresses were sold on Tuesday, 0 were sold on Wednesday, and 5 were sold on Thursday. So, 21 mattresses - 7 - 2 - 0 - 5 = 16.