Pulsating variable star mathematics question

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bcheero
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How do I do this problem. I think you have to use this formula L1/L2 = 2.512^(M2-M1) and then see the relationship with the Stephan Boltzmann Law. I keep getting an answer of 0.1 based on the ratio, but it's not making any sense.

Here's the question. Star P is a pulsating variable star. As it pulses, the apparent magnitude of Star P changes by 5 magnitudes. Assuming that its temperature remains constant as it pulses, what is the ratio of Star P's maximum and minimum radius?
 

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Well you know 5 magnitudes is a factor of 100 in luminosity change. And you know that luminosity is proportional to R^2 at constant temperature. So your answer of 0.1 (really it should be 10 since it asks for the ratio of maximum:minimum) seems correct. Why do you think it doesn't make sense?