When one looks around in everyday life, one sees mathematics everywhere: i.e., when making the right decisions whether to rent or buy a bicycle depending on the circumstances. Mathematics can determine (in case of renting a bicycle) the maximum rental cost to ensure that you will be able to pay the rental cost every month. The period you need the bicycle will also influence the rental cost you are prepared to pay. It is also possible to evaluate whether renting is better than buying, depending on the interest rate the bank is offering you for investing your saved money that you do not spend at once because you rent the bicycle. (Contains 3 figures.)