Ratio of radio active isotopes?

When she asks that, she is either asking in a specific case that you don't mention, so we cannot help, or she is uming that it is understood to mean the natural abundance ratio, in which case you would need to look it up. I suspect the other answers have gotten it right; I only know about ten (at most) element isotope distributions off the top of my head and phophorus isn't one of them.