Samara's income is $30 a month and she spends all of it on music downloads and gasoline. The price of a music download is $3 and the price of a gallon of gasoline is $3. At Samara's best affordable point, her marginal rate of substitution is ________ gallon of gasoline per music download. (萨马拉的收入是每月30美元,她花了所有的音乐下载和汽油。音乐下载的价格为3美元,一加仑汽油的价格为3美元。在萨马拉的最实惠的地方,她的边际替代率是 _ _ _ _ _ _ _ _加仑每个音乐下载的汽油。——中文由在线翻译而来,仅供参考)