Answer:
0.714 liter.
Explanation:
Given:
The balloon initially has a volume of 0.4 liters and a temperature of 20 degrees Celsius.
It is heated to a temperature of 250 degrees Celsius.
Question asked:
What will be the volume of the balloon after he heats it to a temperature of 250 degrees Celsius ?
Solution:
By using:

Assuming pressure as constant,
V∝ T
Now, let K is the constant.
V = KT
Let initial volume of balloon ,
= 0.4 liter
1000 liter = 1 meter cube
1 liter = 
0.4 liter = 
And initial temperature of balloon,
= 20°C = (273 + 20)K
= 293 K
Let the final volume of balloon is 
And a given, final temperature of balloon,
is 250°C = (273 + 250)K
= 523 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, the volume of the balloon be after he heats it to a temperature of 250 degrees Celsius is 0.714 liter.