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
 = 4.3 liter
1000 liter = 1 meter cube
1 liter = 
4.3 liter = 
And initial temperature of balloon,  = 350 K
 = 350 K
Let the final volume of balloon is  
 
And as given, final temperature of balloon,  is 250 K
 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.