Answer:
On second day: 500 cm^3
On third day: 250 cm^3
Explanation:
Let x be the initial amount of helium gas in the balloon.
As it is mentioned in the question that the balloon loses half of it's gas in one day. So after one day the gas will be half of the current volume. And to calculate the volume of gas the next day, the reduced volume will be divided by two.
So,
x = 1000 cm^3
Volume on second day,
x1 = 1000/2
=> 500 cm^3
Volume of gas on third day
x2 = x1/2
=> 500/2
=> 250 cm^3