Without solving the problem, please comment on whether a purchase plan of buying $75$ tons from Russia and $50$ tons from Iran can be an optimal plan.

First check feasibility. According to constraints, it is feasible.

$75+50>=120$

$75>=0\mathrm{.}5(75+50)$

$50>=40$

$75<=100$

But none of the constraints in binding. So$,$ it cannot be boundary solution, so cannot be optimal.