There are a total of 20 + 15 = <<20+15=35>>35 balloons at the party.

Out of the 35, 3 + 2 = 5 balloons burst.

Thus, only 35 - 5 = <<35-5=30>>30 balloons are left.

# Answer

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<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
At a birthday party, there are 20 red balloons and 15 green balloons. Before the party started, 3 red and 2 green balloons burst. How many balloons are left?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
30
At a birthday party, there are 20 red balloons and 15 green balloons. Before the party started, 3 red and 2 green balloons burst. How many balloons are left?
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