6/(6+5+4)=6/ 15=2/5

2.

2/5*5=2 (hours)

3.

3/(2+3+5)=3/ 10

4.

Time spent on flat roads =5/(6+5+4)*5=5/3 (hours)

Horizontal road distance =5/3*4.5=7.5 (km)

Whole journey =7.5/(3/ 10)=25 (km)