A small treat is floating in a rectangular tank measuring 8 inches wide, 10 inches long, and 12 inches high. If the water is currently 5 inches high and a smart crow can reach at most 3 inches down into the tank, at least how many $2 \times 2 \times 2$-inch metal cubes should the crow drop in the tank to get the treat?
Solution A: I assume the tank has a volume of $5 \cdot 8 \cdot 10.$ The crow can reach three inches, so that amounts to a volume of $8 \cdot 8 \cdot 10.$ The total volume is $12 \cdot 8 \cdot 10,$ and I need to add $400 \ \text{in}^3=12\cdot 8 \cdot 10 - 8 \cdot 8 \cdot 10 \ \text{in}^3.$
Meaning: 50 of $2 \cdot 2 \cdot 2 $ cubes.
Solution B:
The height difference: $12-5-3=4$ inches.
To fill $ 4 \cdot 8 \cdot 10$, or $320$ volume, so drop $40$ cubes.