To find out how much of the mystery substance (m) Dr. Burrows added, we can solve the equation:
\[ 13.93 + m = 18.0 \]
To isolate \( m \), we subtract 13.93 from both sides:
\[ m = 18.0 - 13.93 \]
Now, we perform the subtraction:
\[ m = 4.07 \]
Therefore, the amount of the mystery substance (m) that Dr. Burrows added to his experiment is 4.07.