A $6 \mathrm{kg}$ bomb at rest explodes into three equal pieces $P, Q$ and $R.$ If $P$ flies with speed $30 \mathrm{m} {\mathrm{s}}^{-1}$ and $Q$ with speed $40 \mathrm{m} {\mathrm{s}}^{-1}$ making an angle $90°$ with the direction of $P.$ The angle between the direction of motion of $P$ and $R$ is about