A runner is planning for a 10km race. She can maintain a steady speed of 3.9 m/s for as much time as needed before ending the race with a 7.9 m/s…

A runner is planning for a 10km race. She can maintain a steady speed of 3.9 m/s for as much time as needed before ending the race with a 7.9 m/s sprint. If she wants to finish in 40 min or less, how far from the finish should she begin to sprint?