This problem is difficult to understand. What do they mean by 'missing €?" The daughter is lent 100€ and then spends 97€ on a dress, leaving her with 3€. She gives each of her parents back 1€, meaning she's left with 1€. Thus, the current situation is: She has a 97€ dress, 1€ in cash, and a debt of 48€ to each parent. Each parent has 1€ in cash. Therefore, her total assets are equal to the sum of the debts she owes, meaning she has a net worth of 0€ (assuming the dress has not depreciated in value). When the parents' assets are summed with the debt owed to them by their daughter, it equals 100€--the original sum of money.
Yeah it's more of a trick than a question. When they do "49+49=98+my€=99" they confuse the reader by using "+my€" and making them think it's accounted for, where if it represents what she owes then it should be "-my€" and it equals 97, which is how much she spent on the dress so I think the "missing €(I think this is to make people think they got it right with "in her pocket" then think it's "accounted for") is the 97€ she spent on a dress. These questions should be banned from existance since they're not really math, but it's still fun to find the flaw and watch your friends get confused solving it.