To find the amount of the mystery substance \( m \), we can solve the equation:
\[ 13.93 + m = 18.0 \]
To isolate \( m \), subtract 13.93 from both sides of the equation:
\[ m = 18.0 - 13.93 \]
Now, calculate the right side:
\[ m = 4.07 \]
So, Dr. Burrows added 4.07 units of the mystery substance to his experiment.