Respuesta :

maybe 25gs of force because 10+15=25
  
If backpack was simply at rest then normal force would be 100N( g=10m/s^2). It's is being pulled gently in upward direction. So Normal force will be. N= mg - 15 = 10x10 - 15 = 85N. I took g=10m/s^2 for simplification. Hope it helps.