A speeder is pulling directly away and increasing his distance from a police car that is moving at 25 m/s with respect to the ground. The radar gun in the police car emits an electromagnetic wave with a frequency of 7.1 × 109 Hz. The wave reflects from the speeder's car and returns to the police car, where its frequency is measured to be 400 Hz less than the emitted frequency. Find the speeder's speed with respect to the ground.