I'm assuming you mean to say that the clock already showed 10 seconds when she started the race.
It took her 5 seconds to run 100 meters since 15-10 = 5.
To find her speed, divide 100 over 5 to get 100/5 = 20 meters per second
20 meters per second is approximately equal to 44.7387258 miles per hour.
Something seems really off because the fastest record speed for a human runner was around 28 mph. Though of course, this is just a hypothetical math problem.
