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.