Consider a scenario with an objective function: Minimize $$10$X$1+$ $$15$X$2.$ Assume that the value of X$1$ in the optimal solution is zero and the reduced cost for variable X$1$ is $$2.$ If we change one of the objective function coefficients, at what objective function, X$1$ becomes non$-$zero as part of the optimal solution $($in other words: At what objective function, X$1$ becomes cost$-$efficient$)?$