Final answer:
To find out how many of Denis's baseball cards are not new, we first calculate the new cards (80 x 1/4 = 20) and subtract from the total, revealing that 60 cards are not new.
Step-by-step explanation:
The student asked how many of Denis's baseball cards are not new if he has 80 cards and 1/4 of them are new. To find this, we first need to determine how many cards are new by multiplying the total number of cards by 1/4.
80 cards × 1/4 = 20 cards are new. The number of cards that are not new is the remaining portion of his collection.
To calculate the cards that are not new, you subtract the new cards from the total number of cards:
80 cards - 20 cards = 60 cards are not new. So the answer is (C) 60.