以内 (yǐ nèi) means 'within' a limit, often used with time, price, or quantity (e.g., 三天以内 'within three days'). It focuses on the maximum boundary, unlike 里 (lǐ) or 内 (nèi), which generally mean 'inside' a place or container. For mathematical 'less than', use the verb 小于 (xiǎo yú).