According to a model developed by economists E. Heady and Jesek, if fertilizer made from $N$ pounds of nitrogen and $P$ pounds of phosphate is used on an acre of farmland, then the yield of corn (in bushels per acre) is $$Y=7.5+0.6 N+0.7 P-0.001 N^{2}-0.002 P^{2}+0.001 N P$$ A farmer intends to spend $\$ 30$ per acre on fertilizer. If nitrogen costs 25 cents 1$b$ and phosphate costs 20 cents 1 $\mathrm{b}$ , which combination of $N$ and $L$ produces the highest yield of corm?