1. Calculate the acceleration:

* Newton's Second Law: Force (F) = Mass (m) * Acceleration (a)

* Rearranging to solve for acceleration: a = F / m

* Plugging in the values: a = 25 N / 10 kg = 2.5 m/s²

2. Calculate the final velocity:

* Constant acceleration equation: v = u + at

* v = final velocity

* u = initial velocity (0 m/s since the body is at rest)

* a = acceleration (2.5 m/s²)

* t = time (3 seconds)

* Plugging in the values: v = 0 + (2.5 m/s²)(3 s) = 7.5 m/s

Therefore, the velocity of the body after three seconds is 7.5 m/s.