Question 960007
The description is faulty, but the situation is a slightly different example similar to <a href="http://www.algebra.com/algebra/homework/Volume/Volume.faq.question.959400.html">http://www.algebra.com/algebra/homework/Volume/Volume.faq.question.959400.html</a>.
If you can understand that (slightly messy) solution, then you should understand how to solve the problem you posted.