A pre-tensioned rectangular concrete beam $150$ mm wide and $300$ mm depth is prestressed with three straight tendons, each having a cross-sectional are of $50 \: mm^2$, to an initial stress of $1200 \: N/mm^2$. The tendons are located at $100$ mm from the soffit of the beam. If the modular ratio is $6$, the loss of prestressing force (in kN, up to one decimal place) due to the elastic deformation of concrete only is _________.