If you spray 18mg, there are 20% of the bacteria remaining. If you spray another 18mg, it will kill all but 20% of the remaining bacteria, so there will be 20% * 20% = 4% remaining. Let r be the portion of bacteria remaining and x the amount of lysol sprayed. Then
r = (20%)^(x/18mg)
So we want to find the x that makes r=5%. As a check, since 36mg kills just a little too much, your answer should be slightly less than 36mg.
Post again in this thread if you're still having trouble.