Linear Programming

Q. The corner points of the feasible region determined by the following system of linear inequalities: $2x+y \le 10$, $x+3y \le 15$, $x,y \ge 0$ are (0,0), (5,0), (3,4) and (0,5). Let $Z$ = $ax + by$, where $a, b> 0$ Condition on $a$ and $b$ so that the maximum of $Z$ occurs at both (3,4) and (0,5) is