丽莎买 3 个相同的文件夹和 3 支相同的铅笔共支付了 $18.90。霍华德买 2 个这样的文件夹和 1 支这样的铅笔支付了 $10.50。每个文件夹多少钱？

1. Find the cost of 1 file and 1 pencil from Lisa's purchase.

Cost of 3 files and 3 pencils = $\char36{}18.90$
Cost of 1 file and 1 pencil = $\char36{}18.90 \div 3 = \char36{}6.30$

2. Find the cost of 1 file by comparing with Howard's purchase.

Howard bought 2 files and 1 pencil for $\char36{}10.50$.
The difference between Howard's purchase and 1 file + 1 pencil is exactly 1 file.
Cost of 1 file = $\char36{}10.50 - \char36{}6.30 = \char36{}4.20$