Question

A balloon holds 20.0 liters of helium at 10.0°C. If the temperature increases to 50.0°C, and the pressure does

Answer 1

Volum at the beginning: $V_{1}=10$
Volum at the end: 
$V_{2}=V_{1}*(1+ \alpha* (t_{2}-t_{1}))$
$t_{2}-t_{1}=50-10=40$
α for helium = $0,003658\frac{1}{C}$
As the result you get:
$V_{2}=20*(1+0,003658*40)=22,9264 l$
Final volume of the balloon is 22,9264 liters.

(Only if it doesn't explode during being heated ;) just a joke)