Given that the mixture was in a ratio of 3:5, we can set up this fraction:
percent of original amount of mixture evaporated = \(10(\frac{3}{8}*40*0.02+\frac{5}{8}*40*0.05)/40=0.3875=38.75%\)

Note about solving this is that you don't need to know the actual volume of the initial mixture in order to find the solution, since it's asking for a percent of the original, not an actual value. If we weren't given 40 as the initial volume, you could still set up the fraction as:
\(\frac{10(3x*0.02+5x*0.05)}{8x}=0.3875\)