The maximum and minimum resultant of 2 forces acting on a particle are 25N and 5N respectively. Find the Magnitude of the forces.

If the two forces were exactly aligned in direction the magnitide of the resultant is the sum of the two forces, and if one is acting in a direction 180 degrees from the other the resultant magnitude is the difference between them. So now you have two conditions that define two equations with two unknowns. Set up the equations and solve algebraically. What do you get for an answer?