Is it correct to write Na2B2O4 as 2NaBO2? My teacher wrote the reaction for heating of borax like this:
$$\ce{Na2B4(OH)4 .8H2O-&gt;Na2B4O7 + 10H2O}$$
$$\ce{Na2B4O7-&gt;B2O3  +Na2B2O4}$$
And told us that $\ce{Na2B2O4}$ can be written as $\ce{2NaBO2}$,is this correct?
If yes then why?
No, $\ce{Na2B2O4}$ can not be written as $\ce{2NaBO2}$. $\ce{Na2B2O4}$ means 1 mole of $\ce{Na2B2O4}$ and $\ce{2NaBO2}$ means there are 2 moles of $\ce{NaBO2}$. Both are different compounds and have different molar mass. 

Actually in real $\ce{2NaBO2}$ should be written as $\ce{(NaBO2)2}$. Now, both $\ce{Na2B2O4}$ and $\ce{(NaBO2)2}$ are same.