I went to the post office to mail my valentine card.  The postmaster weighed my envelope and said it needed 18 cents in postage.  I looked in purse and found one of each of the following stamps:

• 3 cent stamp
• 5 cent stamp
  
• 6 cent stamp
• 7 cent stamp
• 8 cent stamp

For 4th and 5th graders:  Which stamps would I use if my envelope weighed:
    19 cents ___________________________________________
    20 cents ___________________________________________
    21 cents ___________________________________________
    22 cents ___________________________________________