Find Prism Volume by Packing Practice Test

Maya is building a rectangular storage box using unit cubes. She starts with a base layer that is 4 cubes long and 3 cubes wide. She then stacks 2 more identical layers on top. If she decides to add one more layer but removes 2 unit cubes from the top layer, how many unit cubes will be in her box?

A science teacher shows students two methods to find the volume of a rectangular prism made of unit cubes. Method 1: Count all the unit cubes by packing. Method 2: Multiply length × width × height. The prism measures 7 units long, 3 units wide, and 4 units tall. One student gets 84 cubic units using Method 2 but counts 82 cubes using Method 1. What is the most likely explanation?