Steps and Understanding :
1) By Newtons Second Law of Motion,
Force is rate of change of Momentum,
Here, assuming Force is constant.
Mass of particle, m = 200g = 0.2Kg
Final Velocity, v = 25m/s
Initial Velocity, u = 15m/s
Time taken, t = 2.5s
Rate of change of momentum =m(v-u)/t
F = m(v-u) /t
= 0.2 (25-15)/2.5
Magnitude of Force is 0.8 N .