how do you calculate the isotopic abundance if you have 3 isotopes instead of 2?

Question

anonymous · Answer

what isotopes are you looking at?

anonymous · Answer

Silicon

anonymous · Answer

Do you have the 3 isotope numbers and how often they occur in nature?

anonymous · Answer

can you just give me an example of how to solve this 
i got this question off a friend.

anonymous · Answer

multiply the atomic weight by the percentage for each then add them togeter.

anonymous · Answer

Look at Liliana's question 2 above yours.

jfraser · Answer

@Z, I think he's asking the question in the opposite direction. How do you calculate the percentages of 3 isotopes if you have the average. Short answer is, I don't think you can. Calculating a weighted average works because you can set up a system of 2 equations and 2 unknowns. Working backwards, you'd need a third equation to solve for a third percentage, and I don't konw if you can do that.