a= 7, b= 0, c= 1 -> old= x, new= 1
a=15, b= 0, c= 2 -> old= x, new= 2
a= 7, b= 0, c= 3 -> old= 1, new= 3
a=15, b= 0, c= 4 -> old= 2, new= 4