There are 20 - 2 = <<20-2=18>>18 floors that are each 3 meters high.

So the first 18 floors is 18 x 3 = <<18*3=54>>54 meters high.

The last two floors are each 3 + 0.5 = <<3+0.5=3.5>>3.5 meters high.

So the total height of the last two floors is 3.5 x 2 = <<3.5*2=7>>7 meters.

Therefore, the building is 54 + 7 = <<54+7=61>>61 meters high.

# Answer

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<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
A building has 20 floors. Each floor is 3 meters high, except for the last two floors. The last two floors are each 0.5 meters higher. How tall is the building?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
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user
61
A building has 20 floors. Each floor is 3 meters high, except for the last two floors. The last two floors are each 0.5 meters higher. How tall is the building?
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