4 quarts = 1 gallon.
and every bottle is 1 quart so 4 bottles equals 1 gallon.
with the 4 gallons she has just do 4 times 4 equals 16 so 16 bottles of water for 4 gallons.
now with the .5 gallons left thats half the bottles also so 2 bottles of water.
16+2=18
she can fill 18 bottles of water!!!!!!