Question

Brian bought a 15 pound bag of comcrete. he split the bag into smaller bags that each hold two and a half pounds. how many smaller bags did he fill

Answer 1

6 because 2 and a half + 2 and a half =5 then 5x3= 15

Answer 2

Okay.  You have 15 lbs of concrete, and x bags that can individually hold 2.5 lbs.  What you need to do is divide 15 by 2.5, and the quotient will be 6.
Problem:  15 ÷ 2.5 = 6
So the answer to your problem will be: 6 bags.