A shirt costs 97€ - you borrow 50€ from mum and 50 € from Dad. You buy the shirt with the 100€ and get 3€ change, you give mum 1€ and Dad 1€ and yourself 1€. You now owe Mum 49€ and Dad 49€. 49+49 = 98€ and you still have 1€ which equals 99€ - where has the other 1€ gone to ?
as jeffa says, parents have paid 49 + 49 for the shirt, which you owe them. You also have a euro left in your hand. and if you gave it to one of them it would be 49 and 48 which adds up to the 97 which it cost.
I don't know how else to explain it! You owe each parent €49, so the total you owe is €98. Of that amount, you already have €1, so your "balance" (if you will) is €97, which equals the cost of the shirt.
You originally borrowed 100€. Once you have completed the transaction and bought the shirt, you now have a shirt worth 97€ +3€. You partially repay the debt, giving back 2€, and are left with a shirt worth 97€ in your possession plus 1€ cash. 2+97+1 +100
I think the confusion arose due to the phrasing of the question: "where has the other 1€ gone to?" As has been explained, it hasn't gone anywhere because you still have it!