With the graph attached below you can determine when was the moment that
Sally tripped and fell.
You can also see that they run for 4 minutes before starting to rest. Then, after resting 10 minutes (starting at the 9 min mark and ending at 14) they started running a bit, that's why the line is not at zero anymore. After moving a bit, between 15 and 16 min mark, they stop distancing for one minute. So the answer is: Sally fell at the 15 min mark.
