All in all, the remaining blue pen: 30×9× 18%=48.6 yuan.

Price difference of 54-48.6 =5.4 yuan.

Each red pen can save 5×( 1-0.85)=0.75 yuan.

Generally speaking, each red pen can save 5× 18%=0.9 yuan.

Poor: 0.9-0.75=0. 15 yuan.

I bought a red pen: 5.4÷0. 15=36.