Xiaoming asked Xiaojun to buy 5 mechanical pencils and 8 notebooks on his behalf, and gave Xiaojun 20.4 yuan according to the price, but Xiaojun bought 8 mechanical pencils and 5 notebooks, and got back 1.8 yuan. How much is a mechanical pencil? Yuan?
小明请小君代买5支自动铅笔和8本笔记本，按价钱交给小君20.4元，结果小君却买了8支自动铅笔和5本笔记本，找回1.8元，求一支自动铅笔多少元？
<result>2.4</result>