I--------I--------I--------I--------I--------I traseul
I--------I 1/5 prima zi
I--------I--------I--------I 3/4 din rest
I--------I 15 km - a 4-a zi
traseul 15 x 5= 75 km
prin ecuatie
x=lungimea traseului
x/5 = 1 zi
x-x/5=4x/5 rest
4x/5 * 3/4=3x/5 = a 2-a zi
x/5+3x/5+15=x
x+3x+75=5x
x=75 km