To use a floor cleaner, two thirds of a cup of cleaner must be mixed with one-half gallon of water. If only one-half cup of cleaner is left, how much water should be mixed with it?
1/4 gallon
1/3 gallon
2/5 gallon
3/8 gallon
5/6 gallon
根据题意，设需水量为x，故2/3:1/2=1/2:x，所以x=3/8，故本题选D。