Data:
As the race is an-hour ( 1 h), it is equal to a race of 6/6 hour
Sarah ran 2/6 hour before passing the baton to Jenny:
Substract the fraction Sarah run from the total time of the race (6/6):
After Sarah passed the baton to Jenny the first time it is left over 4/6 hour of the race.
Jenny ran 3/6hour, then passed the baton back to Sarah:
Substract 3/6 from the time left over (4/6):
Then, after Jenny passed back the baton to Sarah it is left over 1/6 hou of the race.
Sarah needs to run for 1/6 hour to complete the race