圆珠笔单价:
7.8-(6.9十7.8)除以7x3
=7.8-2.1x3
=1.5(元)铅笔的单价:

圆珠笔单价:
6.9-(6.9十7.8)除以7x3
=6.9-2.1x3
=0.6(元)