15X8= 120 (points)

120-84=36 (point)

8+4= 12 (point)

36÷ 12=3 channels

Doing it right is 15-3= 12 (Dao)

Equation:

Assuming that x is right, then 15-X is wrong.

8X-4( 15-X)=84

8X-60+4X=84

X= 12