In a Linear Programming Problem, the Binding Constraints for the Optimal

In a linear programming problem, the binding constraints for the optimal solution are
5X + 3Y $\le$ 30
2X + 5Y $\le$ 20
a.Fill in the blanks in the following sentence:
As long as the slope of the objective function stays between _______ and _______, the current optimal solution point will remain optimal.
b.Which of these objective functions will lead to the same optimal solution?
1) 2X + 1Y 2) 7X + 8Y 3) 80X + 60Y 4) 25X + 35Y