ionic activities

Find the activity(not the activity coeefficien)t of the (C3H7)4N+ (tetrapropylammonium) ion in a solution containing 0.005 M (C3H7)4N+Br- plus 0.005 M (CH3)4N+Cl-.

Activity coefficients :

Ionic strength    .001       .005       .01       .05       0.1
(C3H7)4N+ :     .966       .931       .912      .85      .82
CH3)4N+:         .964       .928       .902      .82      .775

The ionic strength is equal to 1/2 the summation of the (concentration of the species multiplied by its charge squared)
Ex: ionic strength of .02 KBr
= 1/2 ([K](1^2)+[Br](1^2))
=1/2((.02*1) +(.02*1))
=.02

Activity=concentration*activity coefficient

The question here is, how do i figure out the concentration od the tetrapropylammonium ion for the concentration in the equation, and how do i figure out the activity coefficient because first the ionic strength will be affected by mixing it in a different solution, and also, the solution has another ion with a different activity coefficient.