Question

The apartments in Ericas apartment house are consecutively numbered on each floor. The sum of her and next door neighbor's number is 2409. What are the two numbers?

Answer 1

Let x be the number of Ericas's apartment, the number of her neighbor will be 
x+1, so

x+ x+1 = 2409

2x = 2408 and x = 1204 and her next door neighbor's is 1205