设铅笔X元,圆珠笔Y元.
5X+3Y=11.5
X+4.2=2Y
5(2Y-4.2)+3Y=11.5
10Y-21+3Y=11.5
13Y=32.5
Y=2.5
X=5-4.2=0.8
答:铅笔0.8元,圆珠笔2.5元.