>>> David has three sisters. Each of them has one brother. How many brothers does David have?

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From your statement, David has three sisters, and each of them is mentioned as having one brother. However, the information is not explicitly clear if the brother is shared among all three sisters or if there are multiple brothers.

If there is only one brother, and it is shared by all three sisters, then David has three brothers. If there are multiple brothers, but the information is not specified how many there are, then David has an unknown number of brothers.

In summary, based on the information provided, we can confirm that David has at least one brother, but the exact number is not specified.