chemistry

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This is a lengthy problem, but I really don't understand it. Any assistance given would be greatly appreciated.

An analysis was done on a rock sample to determine its rubidium content. The rubidium content of a portion of rock weighing .350g was extracted, and to the extracted sample was added an additional 29.45 micrograms of rubidium-87. the mass spectrum of this spiked sample showed a rubidium-87 peak that was 1.12 times as high as the peak of rubidium-85. assuming that the two isotopes react identically, what is the rubidium content of the rock (expressed in parts per million by mass)?

additional info:
rubidium-87 atomic mass=86.909u
% natural abundance=27.83% of all rubidium
rubidium-85 atomic mass=84.912u
% natural abundance=72.17% of all rubidium

I am particularly confused by the peaks aspect of this problem. Does that mean like something with mass or what? Please help. Thanks.

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