If mutton is X yuan per kilogram, beef is (x+8) yuan per kilogram.

1.8(x+8)-4=2x+2

1.8x+ 14.4-4=2x+2

1.8x+ 10.4=2x+2

2x- 1.8x= 10.4-2

0.2x=8.4

x=42

42 yuan for mutton and (x+8) for beef x+8)= 50 yuan.