I : a kg  dupa mutare a - 15
II. a kg  " " " " " " " "" " a + 15Â
(a+15) - (a-15) = a + 15 - a + 15 = 30 kg
I. |__________a__________|..........15..........|
II. |__________|..........15..........|
              |.........................30......................|
           Â