Generally, we use at, in and on when we talk about the location of things.
What do the prepostions in these three things tell us about the locations?
"Meet Simon at the end of the road."
"You left your glasses in the bathroom."
"Is that a spider on the wall?"
At tells us that the following noun is located at a specific point or location. It shows an exact position.
"She's waiting at the entrance."
"He's sitting on his chair at his desk."
"I work at a bank."
In tells us the noun is in an enclosed space (surround or closed off on all sides). Basically, when something is inside something.
In a box.
In a room.
In a country.
On tells us that the following noun is located on a surface. Use on when one thing is attatched to or touching somthing.
On the table.
On the floor.
On the chair.
What's the difference bewteen these two sentences?
She's at the library.
She's in the library.
She's at the library. - the emphasis is on the her location and the type of place she has gone to.
She's in the library. - the emphasis is on the type of building she is in.
Both of these sentences are fine to answer the question, "Where is she?"
Now decide which preposition needs to be used: