Question

Oxidation number of b in nabh4

Answer 1

you can find O. S of any element by using this method

put O.S of b as x

then the sum of the O.S should be equal to the net charge of the molecule

So 
             +1+x+4(1)=0
                 x+5=0
                  x=-5
remember that you have to know the O.S of other elements........