In a warehouse, the number of cartons on the second floor is 80% of the number on the first floor, and the number on the third floor is 50% of the number on the second floor. If the third floor holds 720 cartons, how many cartons are on the first floor?

The third floor holds 50% of 80% = $0.50\times 0.80=40$% of the first floor's count, so 0.40 $xF=720$ and $F=1,800$. Check: first 1800, second 1440, third 720. 1,440 is 720/0.50, the SECOND floor, which stops one step short. 900 divides by 0.80 alone. 2,400 comes from adding the two cuts (20% off then 50% off, leaving a supposed 30%) instead of compounding them.