A level chem help

Can anyone help me answer this question? For some reason I'm really stuck on it:

A sample of Strontium is made up of only three isotopes: 86Sr, 87Sr and 88Sr.
This sample contains 83% by mass of 88Sr
This sample of strontium has Ar = 87.73
Calculate the percentage abundance of each of the other two isotopes in the sample.

I've tried doing a normal percentage abundance calculation (ie percentage x mass number for each isotope/100) but can't work out how you find out the ratio of each isotope.

Thanks!

I've managed to get an answer through doing trial and error.

What must the sum of the other two percentage abundances be?
And then split this number up (e.g. if it were 25% total, you could do 12 and 13), and substitute into your percentage abundance calculation, see if these numbers give you the Ar = 87.73, and if not adjust until you do.