To solve this, it is easiest to set up a proportion, letting x represent the number that we don't know.
2/10 = x/5
Now, we can use cross products to simplify this proportion. Cross products sets the products of the numerator and denominator of separate fractions equal to one another. It results in the equation:
(5)(2) = (10)(x)
Let's multiply out these numbers to further simplify the equation.
10 = 10x
Finally, we have to divide both sides by 10, to get our variable x alone
x = 1
Therefore, 2 tenths is equal to 1 fifth.