I borrow 50k from dad and 50k from mum to buy laptop that costs 97k, I had 3k left.
I returned 1k to dad and 1k to mom, and I reserved 1k for myself. I now owe dad 49k and mom 49k also.
49k +49k =98k plus 1k I reserved for myself, which is 99k.
So where did 1k go...?