This involves a little bit of math.
I wanted to buy a pair of shoes that cost $17, but I had no money. I then borrowed $10 from Juan Dela Cruz and another $10 from John Doe.
I bought the shoes and got $3 as change. I paid Juan Dela Cruz $1, so I owe him $9. I paid John Doe $1, so I also owe him $9. All I have left is $1.
So I owe them both $9 and I have $1. That is $9 + $9 + $1 = $19. So where did the extra $1 go? Isn’t the total supposed to be $20?