Second time: pen 10, Ben 5=30 yuan.

That is, for the second time, I bought five fewer copies of the first book and five more, saving 12 yuan.

That is, each notebook is more expensive than each pen: 12÷5=2.4 yuan.

That is, if you buy a pen 15 for the first time, it is: 42 yuan -24 yuan = 18 yuan, 18 yuan ÷ 15 = 1.2 yuan/pen 1.2 yuan +2.4 yuan =