Your answer is 45 meters.
We can find this because we know that the total length of the track is 100 meters. Since Juliet starts at the 10 meter mark, she only has 90 meters left to run.
Then, she runs for 45 meters, which brings her to a total of 55 meters on the track. This means that to find how much she has left we just do 100 - 55 = 45 meters.
I hope this helps!