Answer:
If the new baseball cards represent 30% of David's special edition baseball card collection, then the original collection represents 70%. Let's represent the total number of cards in David's collection with the variable x. Then we can set up the following equation:
6 = 0.3x
To solve for x, we can divide both sides by 0.3:
x = 6 ÷ 0.3 = 20
Therefore, the total number of cards in David's baseball card collection is 20.