Hey there!
An easy way to look at this is with ratios.
90:30, where it takes 90 seconds per 30 jugs
x:90, where it takes x second per 90 jugs
Figure that if it takes 90 seconds to fill 30 one–gallon jugs, then it will take three times as long to fill 90 one–gallon jugs, since 30 times 3 is 90.
90 seconds × 3 = 270 seconds
It will take 270 seconds, or 270/60 (4.5) minutes to fill 90 jugs. Hope this helped you out! :-)