Question 870282
Boy covered 31 km in 4 hours
He cycled part of the way at 14km/h and walked the rest of the way at 4km/h 
How far did he walk?

let him walk x km
speed =4 km/h
t=x/4

he will cycle 31-x
speed =14km/h
t=(31-x)/14

cycle time + walking time = 4

x/4 + (31-x)/14 =4

Multiply by LCM 28
7x+2(31-x)=4*28
7x+62-2x=112
5x=112-62
5x=50
x=10 km
he walks 10 km