With usual notations the depth of the neutral axis of a balanced section, is given by

ANS:D -

n=critical neutral axis
m=modular ratio
c=permissible stress in concrete
t=permissible stress in steel.

We know that n=[mc/(mc+t)] *d.
= n/d=mc/(mc+t).

Now reciprocating both sides,
= d/n=(mc+t) /mc,
= d/n=1+t/mc,
= d/n-1=t/mc,
= (d-n)/n=t/mc.

Now reciprocating again to get the required expression i.e.
= n/(d-n) = mc/t.