Hi,
D = r*t, distances the same, 13/4 hr = 3.25hr
Let t and (3.25hr-t) represent the times at 8kmph and 5kmph respectively
8kmph*t = 5kmph*(3.25hr-t)
solve for t
13kmpht = 5kmph*3.25hr
t = 5*3.25/13kmph
t = 1.25hr
8kmph*1.25hr = km, the distance walked at 8kmph
The total distance walked entirely = 20km