You used Rs.300 and now you have Rs.200 with you. You have given Rs.100 to each. So, you have to pay Rs.1300 to them.
So, don't add USED MONEY and MONEY YOU HAVE TO PAY.
Just, simply add MONEY YOU PAID and MONEY YOU HAVE TO PAY,
i.e., 1300+200 =1500.
Simply, I want to tell you that you can not only use 300 but also 310 or 350 or 400. If you have Rs 500 note.
So, friend don't get confused and keep your mind fresh.