Since there are 20 cards in a pack and 52 weeks in a year, Phil starts by collecting 20*52= <<20*52=1040>>1040 baseball cards

He then loses half of them for 1040/2= <<1040/2=520>>520 cards remaining.

# Answer

520
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Phil likes to collect baseball cards.  He buys a pack of twenty each week for a year, but then loses half of them one day in a fire.  How many baseball cards does Phil have left?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
520
Phil likes to collect baseball cards.  He buys a pack of twenty each week for a year, but then loses half of them one day in a fire.  How many baseball cards does Phil have left?
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