[ Semi-invariants $]$ [ $\left[\begin{array}{l}\text { Processes and operations }\end{array}\right]$

A chocolate bar has a size of $4 \times 10$ squares. In one move, it is allowed to break one of the existing pieces into two along a straight line. What is the minimum number of moves required to break the entire chocolate bar into pieces of one square each?

#
After each break, the number of pieces increases by 1.

## Solution

Notice that after each break, the number of pieces increases by 1. Initially, we have a whole chocolate bar, i.e., one piece, and in the end, we need to get 40 pieces of one tile each. Therefore, we need to perform exactly 39 breaks.

## Answer

In 39 moves.