Jeff would have had 10 cards to begin with.
Then, if Jeff sold half of his baseball cards, he would then have 5 cards.
It says that Jeff bought 16 more cards, so 16 + 5
Well, 16 + 5 = 21. The text says that he now has 21 cards, so this should be a correct answer.
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Joke: What is a snowman's favorite type of Mexican food?
Answer: Burr-itos!
Have a great day and God bless! :D