A resultant force of 15.0N acts on a body of 4s, mass 4kg. Calculate the momentum

force * time = impulse = change of momentum

15 * 4 = 60 kg m/s

or use second law directly
F = m a
a = 15 N/4kg = (15/4) m/s^2
v = a t = 15/4* 4 = 15
p = m v = 4 * 15 = 60 kg m/s