Describe the relationship between speed, distance, time of travel and interpolate time to travel 8m for SLOW walk and extrapolate for 25m FAST...

First you must graph your data correctly.  Make a graph
with Time along the X-axis and Distance along the Y-axis.  Your three points for the
Slow Walk should be (13.60s, 15m) (26.66s, 30m) (39.93, 45m).  Put your three data
points on the graph for you Slow walk.  Draw a STRAIGHT line that comes as close to all
three points as possible (do not play "connect the dots"--one line that is the best you
can do).  Make sure you extend your line well past your three data points in both
directions.  To "interpolate" for 8.0 meters, go up the Y-axis to where 8.0 meters is,
go over to the line you drew, then go straight down to the X-axis.  The value of the
X-axis at this point is the time you are predicting it would have taken you to go 8.0
meters.

Do a similar graph for your Fast Walk.  Your three
points should be (8.61s, 15m) (16.5s, 30m) and (24.66s, 45m).  Plot these on the graph
and connect them with a straight line.  Extend the line diagonally up toward the right
of the graph until you go past 25 meters.  To extrapolate, go up the Y-axis to where 25
m should be.  Go straight across to the line you drew for the Fast Walk, then go
straight down to the X-axis.  This value is your predicted time to walk 25 meters at a
fast walk.

You need to do the Medium Walk as well. 
However, you did not supply the data.  To do it you must figure out the accumulated time
for each 15 meters you walk. (first time, first 15m) (first time + second time, first
15m + second 15m) and (first time + second time + third time, first 15 m+ second 15m +
third 15m).  Draw the best line.  You should be done.