Sodium oxide can be formed by burning sodium in air using the following chemical reaction:
`2 Na + 1/2 O_2 -> Na_2O`
i.e. 2 moles of sodium will produce one mole of sodium oxide.
The atomic weight of sodium is 23 and that of oxygen is 16. Hence the atomic weight of sodium oxide = 2x23+16 = 62 gm/mole
From the chemical reaction: 2 moles of sodium form 1 mole of sodium oxide.
or 2x23 gm sodium forms 62 gm sodium oxide
or, 46 gm sodium forms 62 gm sodium oxide
or 1 gm sodium forms 62/46 gm sodium oxide
or, 20 gm sodium will form 62x20/46 gm = 26.96 gm sodium oxide
Hope this helps