Final answer:
The combined support force the floor exerts on your feet when standing still with your niece sitting on your shoulders is 530 Newtons, which is the sum of your weight and your niece's weight.
Step-by-step explanation:
To calculate the support force the floor exerts on your feet when you and your niece are stationary, you must add both your weight and your niece's weight. This is a physics problem involving Newton's Third Law of Motion, which states that for every action there is an equal and opposite reaction. Thus, if you are not accelerating, the support force (normal force) exerted by the floor will be equal to the combined weight of you and your niece.
Your weight: 380 Newtons
Your niece's weight: 150 Newtons
Total weight: 380 N + 150 N = 530 Newtons
Therefore, the support force the floor exerts will be 530 Newtons.