Answer: B: 6 feet
Step-by-step explanation: To solve this problem, we need to use the concept of leverage, which is the force applied to an object at a distance from its pivot point. In this case, Carlos is applying a force of 3 feet from the fulcrum, while his baby sister is applying a force 3 times as large, at a distance from the fulcrum.
To find the distance from the fulcrum that the baby sister must sit, we need to balance the forces applied by Carlos and his sister. This means that the product of the force applied by Carlos and the distance from the fulcrum must be equal to the product of the force applied by his sister and the distance from the fulcrum where she must sit.
We can use this information to write the equation: 3 * 3 = 3 * x
Where x is the distance from the fulcrum where the baby sister must sit. We can solve for x by dividing both sides of the equation by 3 to get: x = 6
Therefore, the baby sister must sit 6 feet from the fulcrum in order to balance the forces applied by Carlos and his sister. The correct answer is B. 6 feet.