If the 3/5 was a whole number, it would all be simple. We would just put the number above 4 and divide. Easy, right?
Well, you do basically the same thing. Since having fractions over fractions confuse me, I'm going to put 3/5 into a decimal.
3/5=6/10=0.6
Now, I put that over 4, because there are 4 bottles.
0.6/4=6/40=3/20
She put 3/20 gallons into each bottle. Hope this helped!