I have struggled with this problem and I believe there is an inherent fallacy in the problem. For example, if the reaction is
A + B ==> C, then
K = (C)/(A)(B) = 4.2
BUT, by Le Chatelier's Principle, if we increase A and/or B, the reaction will be shifted to the right and more C will be formed.
Try it with 1 M A and 1 M B and you end up with 0.617 C and 1-0.617 = 0.383 A and B for a yield of 61.7%(100*0.617/1).
With 2 M A and 2 M B, we end up with 1.42 C and 2-1.42 = 0.58 A and B for a yield of 71% (100*1.42/2).
With 3 M A and 3 M B, we end up with 2.265 C and 3-2.265 = 0.734 for a yield of 73.4% (100*2.265/3).
So the percentage changes because the ratio changes even though I started with equimolar amounts of A and B.
Working backwards.
A + B ==> C
Start with 1 M A and 1 M B, you can't get 4.2 for K if 67% is used for the yield (if I understand what equilibrium yield is) but 4.2 is obtained if we use 61.7%. The same is true for the other amounts of A and B initially.
If you find that equilibrium yield is something different than (C)/(A)at equilibrium please let me know.