No need to get bogged down in confusing terminology here. Start with the basics: dB = 10*log(I/I0). If you know an equation, it's probably easiest to start there. Here, it's useful to know that sound intensity is measured on a log scale.
Now, it's asking you the ratio of the minimum sound intensities heard by two different individuals. We're using "I" to represent intensity here. In other words, it's asking you for the ratio I(2)/I(1), where 1 and 2 denote the people. db for individual 1 = 20 dB and dB for individual 2 = 40. Solve for I(2)/I(1).
20 = 10*log(I(1)/I0)
40 = 10*log(I(2)/I0)
2 = log(I(1)/I0)
4 = log(I(2)/I0)
I don't like working in logs, so let's get rid of the logs:
10^2 = I(1)/I0
10^4 = I(2)/I0
Divide these by each other:
10^4/10^2 = [I(2)/I0]/[I(1)/I0]
10^2 = I(2)/I(1)
Therefore, the ratio of one to the other is 100:1 or 1:100.