UpdatePlayerExpAndLevel.cs

public void UpdatePlayerExpAndLevel(int gainedExp) 
{ 
  double newNeededExp = this.ExperiencePoints; 
  double newMaximumHitpoints = this.MaximumHitPoints; 
  double multiplier;

  this.ExperiencePoints += gainedExp;

  if (this.ExperiencePoints >= this.NeededExperiencePoints) 
  { 
    this.Level += 1;

    if (this.Level <= 3)
    {
      multiplier = 1.35;
    }
    else if (this.Level <= 6)
    {
      multiplier = 1.25;
    }
    else
    {
      multiplier = 1.15;
    }

    this.ExperiencePoints -= this.NeededExperiencePoints; 
    newNeededExp *= 1.15; 
    this.NeededExperiencePoints = (int)newNeededExp; 
    newMaximumHitpoints *= multiplier; 
    this.MaximumHitPoints = (int)newMaximumHitpoints; 
    this.CurrentHitPoints = this.MaximumHitPoints; 
  }
}