x= traseul
I zi ........... 1/5 × x
II zi ..........2/5 × ( x - 1/5 × x)
III zi ......... 20 km
x = 1/5 × x + 2/5 × ( x - 1/5 × x) + 20
x = x/5 + 2/5 × ( x - x/5 ) +20
x = x/5 + 2x/5 - 2x/25 + 20
x = 3x/5 -2x/25 + 20
-20 = 15x/25 - 2x/25 -x
-20 = 13x/25 -25x/25
-20 = -12x/25
-12x = -20 × 25
-12x = -500
x = 125/3