Material mix variance

I can understand your confusion, but you have misunderstood what is typed in the answer. What it is saying is that the mix variance and the usage variance will equal each other (as stated in the third sentence. The usage variance equals the mix variance plus the yield variance. Since the yield variance is zero then the usage variance must be equal to the mix variance.

I can see why you thought that was what it was saying, but I hope it is clear now :-)