To determine after how many days the library will have at least 15 people signed up for the event, we start with the current number of sign-ups and the expected increase:
- Currently, 3 people have signed up.
- The library expects 2 more people to sign up each day.
If \(d\) represents the number of days after which we are evaluating the total sign-ups, the total number of sign-ups after \(d\) days can be expressed as:
\[ 3 + 2d \]
We want to find when this total will be at least 15. Therefore, we set up the inequality:
\[ 3 + 2d \geq 15 \]
This is the inequality that can be used to determine how many days it will take for the library to be sure it can run the event.
Therefore, the correct response is:
2d + 3 ≥ 15, where d is the number of days.