Final answer:
To find out the total number of cards in the special collectors edition set, set up a proportion. Subtract the number of cards already collected from the total to find out how many more cards are needed.
Step-by-step explanation:
To find out how many cards are in the special collectors edition set, we can set up a proportion. Let x represent the total number of cards in the set. We can write the proportion as: 18/x = 40/100. To solve for x, we cross multiply and get: 18 * 100 = 40 * x. Simplifying further, we have: 1800 = 40x. Dividing both sides by 40, we find that x is equal to 45. Therefore, there are 45 cards in the selection.
To find out how many more cards Robert needs to collect, we can subtract the number of cards he already has (18) from the total number of cards in the selection (45). Therefore, Robert needs 45 - 18 = 27 more cards.