You do 12*3 because if 1 sheet of poster board can make 12 post cards, and you have 3 poster boards you will need 3 lots of 12 which is equivalent to 3*12!
Josh's estimate IS reasonable because when you have a number 5 or above after the decimal, you round up. Although it's reasonable to round up to 5 pounds, the actual amount is 4.5 pounds, or four and a half.
