Hi. I'm stumped on something in my homework that I feel like shouldn't be a major obstacle.

The problem takes the following form:

Calculate the Ksp for the Y-substituted crystal X_{.4}Y_{.6}Z given the Ksp values for:

XZ -> X^{+} + Z^{-}

YZ -> Y^{+} + Z^{-}

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I can't figure out if I'm supposed to be doing some Hess's law-style combination of the two constituent solubility products, or if there's another approach that I'm missing altogether.

I understand that the Ksp of the substituted crystal should be [.4X]^{.4}[.6Y]^{.6}[Z] by doing an intial-change-equilibrium table, but again, I don't understand how to arrive at a numerical value. Thanks!