to make the money less one of the freinds will carry 9kg and the other 12 kg the last one carries the left luggage weighing 7kg and 6kg which add up to 13kg.
the first freind won't pay anything as he is passing 9kg luggage
the second will pay : 12-10 =2 so 2kg*5$=10$
the last freind will pay 13-10=3 so 3kg*5$=15$
to find the total money the pay we will add 10$ and 15$ which became 25$.
