Genetics question - Penetrance

[Typo]

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This is from the 1996 exam, micro/path section:

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6. An autosomal dominant trait showing 50 percent penetrance will be phenotypically expressed in what percent of the offspring?

A. 0
B. 25
C. 33
D. 50
E. 75
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The answer on the key is B, 25%. I'm having a hard time seeing how you can predict the phenotypic expression of this trait in offspring when you don't know the genotypes of the parents. Even if we assume a cross between two heterozygotes, the penetrance would cause 37.5% to be expressed, right? Anyone have an explanation?

 

[wireless_doc]

Because the penetrance is 50%, the genotypes of the parents have to be Aa and aa. This will result 50% of the offspring with the dominant trait. Again, since the penetrance is 50%, you need to multiply the 50% of the offspring by the penetrance which will give you 25%. I may be wrong, but I think that's how it's solved. Let me know if my reasoning is wrong.