Use reasoning to lead to the answer of the following question:
There were 21 dollars in the cookie jar. Doris spent $6 from the cookie jar. Martha spent half as much as Doris. How much money, in dollars, was left in the cookie jar?

12
Martha spent 6 / 2 = 3 dollars.

Martha and Doris spent 6 + 3 = 9 dollars.

There were 21 - 9 = 12 dollars left in the cookie jar.

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The answer: 12