First you have to write the equation. in the scenario, use standard form. Ax+By=C plug the numbers in. A=2.50, B=1.25, and C is the total, 356.25. the 180 doesn't come in quite yet. your equation is 2.50x+1.25y=356.25. now, since they only bought 180 items, you can't go past that.
I am sorry, but I am about to leave for school, and therefore do not have enough time to answer the last of your question. I hope the part I could answer has helped you.