Answer:
Using my (not very precise) periodic table, I get the mass of Ag as 107.87
Explanation:
This must be a combination of these two isotopes. So the % of the other isotope must be 48.16%.
To work with these percentages in an equation I'll use them as decimals.
So now I can start putting them into an equation:
107.87 Â = Â ( 106.905 â‹… 0.5184 ) Â + Â ( x â‹… 0.4816 ) Â
Because both of the isotope masses, multiplied by their percentage, would give us the overall average mass.
107.87  =  55.419552  ⋅  (  x  ⋅  0.4816 ) Â
Worked out the brackets that we can work out, and then rearrange to give:
107.87  −  55.419552  =  (  x  ⋅  0.4816  )
Again rearrange:
52.450448 =  (  x  ⋅  0.4816  ) Â
And final rearranging:
x  =  52.450448  - 0.4816
x
=
108.909
(using same sig figs as given in question)
You probably want to work through that again using your periodic table value for Ag.