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On the first day, a patient takes one third of all pills from a container. On the next day, the patient takes one third of pills that are left in the container. All together, he took 15 pills from the container.  How many pills there where in the container to begin with?

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Answer: 27.
Let X be the number of pills in the container. Then, on the first day the patient takes 1/3X pills. On the second day the patient takes 1/3(X-1/3X) pills. Therefore, we have 1/3X + 1/3(X-1/3X) = 15. Solving for X, we get X=27.

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