Martha has 18 crayons. She lost half of them, so she bought a new set of 20 crayons. How many crayons in total does Martha have after the purchase? Let's think first. Chain of thought:
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In the beginning, Martha lost 18 / 2 = 9 crayons. So after she bought a set of new crayons, she has in total 9 + 20 = 29 crayons.
Therefore, the answer is 29.
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