The cross-section at mid-span of a beam at the edge of a slab is shown in the sketch. A portion of the slab is considered as the effective flange width for the beam. The grades of concrete and reinforcing steel are $M25$ and $Fe415$, respectively. The total area of reinforcing bars $(A_s)$ is $4000 \: mm^2$. At the ultimate limit state, $x_u$ denotes the depth of the neutral axis from the top fibre. Treat the section as under-reinforced and flanged ($x_u > 100$ mm).

The value of $x_u$ (in mm) computed as per the Limit State Method of $IS \: 456:2000$ is