Answer: ≈ 40.8 kilograms (89.9 pounds)
Explanation: Well, assuming the boy is on Earth, then the gravitational acceleration is approximately 9.8 m/s^2.
So, we get: mass = weight / gravitational acceleration
Put two and two together and you get 400-N / 9.8 m/s^2 ≈ 40.8 kg (89.9 pounds).