To answer this question, two equations are needed.
F = m * a (force = mass * accelaration)
Vf =Vi + (a * t) (final velocity = initial velocity + (accelaration * time)
Knowing that the force is 100 N and the mass is 50 kg we can use the first equation to solve for the acceleration which would be 2 m/s^2.
Plugging in all we know to the second equation we get:
150 m/s = 100 m/s + (2 m/s^2 * t)
This gives us t = 25 seconds