Find the amount of cytosine

I thought that we could calculate the amount of one base present if at least one percentage is given. Exam Krackers FL #2 Q #168 (last q of the third section) says that it cannot be determined. I was wondering if there was particular reason that it could not be determined in this case. I would have guessed 25% cytosine given that adenine is 25%.

The question is:
The section of VEGFR mRNA that anneals to the siRNA contains 25% adenine. What is the cytosine content of the siRNA sequence?...

Find the amount of cytosine