To find the value of the mystery substance (m) in the equation \( 13.93 + m = 18.0 \), we can isolate \( m \) by subtracting 13.93 from both sides of the equation:
\[ m = 18.0 - 13.93 \]
Calculating this gives:
\[ m = 4.07 \]
Thus, the amount of the mystery substance (m) that Dr. Burrows added to his experiment is 4.07.