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Nancy did an experiment. She inserted a paper straw vertically into the bottom of a bottle of black ink. Then, she found that the length of the black part of the paper straw was exactly $12$ cm. She turned the straw upside down and inserted the other end vertically into the bottom of the bottle. She found that the length of the part not colored of the straw was exactly half of that of all the black parts. The paper straw was~\uline{~~~~~~~~~~}~cm. 
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The black parts of the straw was $$12+12=24$$ cm, so the part not colored was $$24\div2=12$$ cm. Thus, the length of the straw was $$24+12=36$$ cm. 