Volum at the beginning: [tex]V_{1}=10[/tex]
Volum at the end:
[tex]V_{2}=V_{1}*(1+ \alpha* (t_{2}-t_{1}))[/tex]
[tex]t_{2}-t_{1}=50-10=40[/tex]
α for helium = [tex]0,003658\frac{1}{C}[/tex]
As the result you get:
[tex]V_{2}=20*(1+0,003658*40)=22,9264 l[/tex]
Final volume of the balloon is 22,9264 liters.
(Only if it doesn't explode during being heated ;) just a joke)