X - drumul total
1) 30/100 • x = 3/10 • x
2) 2/5 • (x - 3/10 • x) = 2/5 • ( x - 3x/10 ) = 2/5 • 7x / 10 = 7x / 25
3) 42 km
3/10 • x + 7x/25 + 42 = x
3x / 10 + 7x / 25 + 42 = x
15 x +14 x + 2100 = 50 x
29 x + 2100 = 50 x
21 x = 2100
X = 100
Raspuns : 100 km