Calculating ratio if relative abundances

Edit: 'of' relative abundances not 'if'

a) A mass spectrum of a sample of indium showed 2 peaks at m/z = 113 and m/z = 115. The relative atomic mass of this sample of indium is 114.5. Use this data to calculate the ratio of relative abundances of the 2 isotopes.

How would I work out the answer to this?

Thank you,

assign the relative abundances as 'x' for m/z 113 and (100-x) for m/z 115

can you see how to proceed from here?

I did manage to get the answer. May I ask how we know the total is 100? The sum in a mass spectrometer varies; it could be any number.

thanks!

The relative abundance is as a percentage, that's why we know it is 100

Is it always as a percentage?

The relative abundance yes, but mass spec data may be presented as peak intensity values in which the most abundant ion is given a value of 100. You then use the data to work out percentage abundances.

When you have relative abundance at 6 and 10, you'd find the product with the mass to charges restively and divide by 16. What I'm confused about is that there is no mention of 100 in the question. There could be small abundances of each to give the relative atomic mass.