(2)y B =(25× 10+5x)×0.9 = 225+4.5x
(3)YA-YB =(200+5x)-(225+4.5x)= 0.5x-25。
When 0.5x-25 < 0, that is, when X < 50, Y A < Y B, so when X < 50, it is more economical to choose the priority method of A;
When 0.5x-25=0, that is, when x=50, Y A =y B, so when x=50, both parties pay in the same way;
When 0.5x-25 > 0, that is, when X > 50, Y A > Y B, so when X > 50, it is more economical to choose the second priority.