Find the oxidation number of the underlined atoms.
`Na_2underlineB_4O_7`

Explanation

`{:( Na_2underline B_4O_7,Na_2,B_4,O_7), (Let ON is ,+1,x,-2):}`
`thereforei.e., (+1 times2) +( x times 4) +(-2 times7)=0 implies2+4x-14=0 implies 4x=12 implies x=+3`
Oxidation number of B in `Na_2B_4O_7` is +3.