7 + 3 = 10 those are the 2 addens
N+7 - 7 more songs added
n-2 - 2 songs deleted (subtracted)
4n - 4 times more songs than before
n/2 - the number of songs cut in half (divided by 2)
0.015 or 3/200
15 divided by 1000 is 0.015
15/5 is 3 1000/5 is 200
This formula solve by discriminant (D=b^2-4ac)
