Answer:
Take a glass, and fill it completely with water.
Now, pour that water in a marked bowl and measure how much water is in the bowl, that amount of water will be 100% of the total water that the glass can have.
Now you need to pour 3/5 of the water back in the glass, it is easier for water until the water in the bowl is exactly 2/5 of the initial amount.
You can do this as follows.
2/5 = 0.4 or 40% in percentage form.
Then if the amount of water in the marked bowl is A, the amount of water that must remain in the marked bowl is A*0.4
Then drop the water in the glass until the water in the marked bowl is the quantity that you found previously.
In this way, you can be sure that the glass is 3/5 full of water.
