Why credits and risks in math classes

Unfortunately, in Austria adolescents or young adults can't handle money in a decent way. Some studies state that 14% of all visitors coming firstly in contact with the debt advice service are 25 years old or younger. Actually situations become worse by taking a closer look, persons of not full age hardly seek advice at an official service institution. They are used to ask their friends or parents for some financial support. A smart-phone-contract is the most common way for indebtedness in the youth, closely followed by a car purchase. All these mentioned forms of borrowing money, even borrowing money from your parents, are kinds of credits (see http://orf.at/stories/2208386/2208367/). The author concentrates in his Ph.D.-thesis on financial topics, which should be taught in school, focus on secondary education. He creates central ideas of financial mathematics  fundamental ideas in the sense of \textsc{Bruner} (see \cite{brj}) respectively, via an empirical study. Three of these central ideas are "Use of stochastics in a finance mathematical context", "time value of money" and "handling of risk". The following modelling exercise is based on these mentioned ideas. \newline    The Austrian school curriculum stipulates the discussion of credits in class. Furthermore in Austria there are some special schools, like "Handelsakademie" (commercial high-school), which deal with  credits in detail. However, all considerations in school are done rather statically. Changes of the interest rate are rarely discussed. In addition, in a bank consultation the assistant usually shows only one credit scenario which is in favour of the debtor, though this is not the only possible scenario. From the point of view of banks it is obvious, they want to realize profit. So, the credit user must have such knowledge in such a consultation situation. At this point we explicitly emphasize: We don't want to put the reader/resp. a pupil off raising a credit, but he/she should be able to understand the behaviour and the dynamics of a credit and to be aware of its risks.