Answer:
2.87 liter.
Explanation:
Given:
Initially volume of balloon = 4.3 liter
Initially temperature of balloon = 350 K
Question asked:
What volume will the gas in the balloon occupy at 250 K ?
Solution:
By using:

Assuming pressure as constant,
V∝ T
Now, let K is the constant.
V = KT
Let initial volume of balloon ,
= 4.3 liter
1000 liter = 1 meter cube
1 liter = 
4.3 liter = 
And initial temperature of balloon,
= 350 K
Let the final volume of balloon is
And as given, final temperature of balloon,
is 250 K
Now, 



Dividing equation 1 and 2,

K cancelled by K.
By cross multiplication:

Now, convert it into liter with the help of calculation done above,

Therefore, volume of the gas in the balloon at 250 K will be 2.87 liter.