Calculate the mass of sodium oxide, which must be extracted to water with a volume

Calculate the mass of sodium oxide, which must be extracted to water with a volume of 3.00 cm³ in order to obtain a solution with a mass fraction of alkali equal to 5.00%.

In order to solve this problem, we perform the following actions:
Na2O + H2O = 2NaOH
We know the following formula:
W r.v = m r.v / mr-ra × 100%
Where mr-pa = m r.v + m H2O
Next, we take the mass of the soluble substance through x, Substitute it into this formula and get the following:
0.05 = x / (x + 3)
Then we solve as follows.
0.05x + 0.15 = x
0.95x = 0.15
Next, we calculate the value of the number x.
x = 0.16 g
m (NaOH) = 0.16 g
M (NaOH) = 23 + 16 + 1 = 40 g / mol
n (NaOH) = m / M = 0.16 / 40 = 0.004 mol
It turns out that n (Na2O) = n (NaOH) / 2 = 0.002 mol
M (Na2O) = 23 × 2 + 16 = 62 g / mol

m (Na2O) = n × M = 0.002 × 62 = 0.124 g
Answer: the mass is 0.124 grams