The initial momentum of the 25 kg object is 25 kg * 12 m/s = 300 kgm/s. After the collision, the momentum of the 25 kg object is 25 kg * 8 m/s = 200 kgm/s. According to the conservation of momentum, the momentum lost by the 25 kg object is equal to the momentum gained by the box. Therefore, the resulting momentum of the box is 300 kgm/s - 200 kgm/s = 100 kg*m/s.