Maximize: $$f(x,y,z)=600x+480y+440z,$$
subject to these four constraints: 
$$x+0.75y+z\leqslant 120$$
$$x\leqslant 50$$
$$y\leqslant 40$$
$$z\leqslant 60$$
Any help or hints on how to solve this would be greatly appreciated! Thank you very much! ☆