Answer:
3/10 liter
Step-by-step explanation:
Assuming your description means that 4/3 liters of punch includes 2/5 liters of water, then the fraction of punch that is water is ...
(2/5)/(4/3) = (2/5)(3/4) = 3/10
3/10 of a liter of water is used in each liter of punch.
