Answer:
20N
Explanation:
Force applied by Samantha = 10N
Force applied by Emily  = 10N
Since both Samantha pushes the box of textbook in the same direction; their forces are additive.
  Net force applied on box  = Force applied by Samantha + Emily
Force is a pull or push on a body
 Net force  = 10N + 10N  = 20N