Given the rationale, provide a reasonable question and answer. Step-by-step reasoning process: 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.
 The question and answer:
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?
The answer is 12