Force pairs:

We must always write about forces this way:

There is a _______________ force exerted by_______________ on ______________

We considered the student sitting in a chair and the chair on the ground.

Then the free-body diagrams look like:

Forces acting on the student:

There is a gravitational force exerted on the student by the Earth (W1).

Its pair is a gravitational force exerted on the Earth by the student.

There is a normal force exerted on the student by the chair. (n1)

Its pair is a normal force exerted on the chair by the student (-n1 in the second free-body diagram.)

Forces acting on the chair:

There is a gravitational force exerted on the chair by the Earth (W2).

Its pair is a gravitational force exerted on the Earth by the chair.

There is a normal force exerted on the chair by the student. (-n1)

Its pair is a normal force exerted on the student by the chair (n1 in the first free-body diagram.)

There is a normal force exerted on the chair by the floor. (n2)

Its pair is a normal force exerted on the floor by the chair.

If neither object is accelerating, then the magnitude of n1 is W1 and the magnitude of n2 is W2+n1=W2+W1.