To make a fixed amount of a dilute solution from a stock solution,
use the formula: C1V1 = C2V2
where: V1 = Volume of stock solution needed to make the new solution.
C1 = Concentration of stock solution.
V2 = Final volume of new solution.
C2 = Final concentration of stock solution
2 * 10 = 0.5 * V2
20 = 0.5V2
$\frac{20}{0.5}$ = V2
= 40dm${}^3$
There is an explanation video available below.