The solubility product constant?

Question

The solubility product constant of AgCl is 1.8 x 10^-10 and that of AgBr is 3.3 x 10^-13. Solid AgNO3 is slowly added to a solution containing 0.0010 M each of NaCl and NaBr. What concentration of Br- would remain in solution when AgCl beings to precipitate?

The solution only gave an answ of 1.8 x 10^-6 but there was no explanation...

Answer 1

AgCl --> Ag+ & Cl-

Ksp = [ Ag+] [Cl-]

1.8e-10 = [ Ag+] [0.0010]

when AgCl begins to ppt, the Ag+ has risen to 1.8e-7 molar

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find the Br- when the  Ag+ has risen to 1.8e-7 molar

AgBr --> Ag+ & Br-

Ksp = [ Ag+] [Br-]

3,3e-13 = [ 1.8e-7] [Br-]

Br- = 1.8 e-6 molar