a = lungimea traseului
b = a/2 = distanta parcursa in prima zi
c= b/2 = distanta parcursa in a doua zi
d= distanta parcursa in ultima zi = 5 km
b+c+d= a
c= a/2/2
a= a/2+a/2/2+5
a-a/2-a/2/2=5
2a/2-a/2-a/2/2=5
a/2-a/2/2=5
a/2-a/2*1/2=5
a/2-a/4=5
2a/4-a/4=5
a/4=5
a=5*4
a=20