Equation 4/5(80+X)= 100-X can be obtained.

4/5*80+4/5*X= 100-X

64+4/5x= 100-X

X+4/5X= 100-64

9/5X=36

X=36*5/9

X=20

2,120 ÷ (1-50%-30%) = 600 (m)

3. 18÷20=0.9 km

18 ÷ 30 = 0.6km.

18-0.6*20=6 (km)