2020
Problem - 4731
In the plane figure shown below, $3$ of the unit squares have been shaded. What is the least number of additional unit squares that must be shaded so that the resulting figure has two lines of symmetry?
Answer
$7$
The crucial observation is that a line of symmetry must pass through the center. This means that every shadowed square must have corresponding mirroring squares with respect to the center. A few error-and-trial can reveal that a minimal of $\boxed{7}$ additional squares need to be shadowed in order to meet the requirement.
