Alex,

If one has a certain fraction, X, survive after 5 minutes, then after 40 minutes one has X^8 of the original population present.

(say a tenth survive 5min of exposure, in the *next* 5 min one in ten of that first 1/10 will survive, and so on)

So, as logarithmic ratios, if you have Y (= -Ln X) after 5 minutes, you'll have 8Y after 40 min.