Answer:
Before, he had 21 baseball cards and 12 football cards.
we know that:
21 = 7*3
12 = 4*3.
He decided to pack the cards in groups of 3, because 3 is the only common factor between 21 and 12.
Now, if he decides to keep one baseball card, now he has:
20 baseball cards and 12 football cards.
Now he has two possibilities for how many cards are in each pack, this is because:
12 = 4*3 = 2*2*3
20 = 4*5 = 2*2*5
12 and 20 have two common factors, 4 and 2.
a) Then he can sell them in packets of 2 cards each with:
12/2 = 6 packs of football.
20/2 = 10 packs of baseball.
b) Or he can sell them in packets of 4 cards each with:
12/4 = 3 packs of football.
20/4 = 5 packs of baseball.
