A Challenging Problem of Solid Angle by HCR

The diagram above shows a hollow circular section having its center O, inner radius $r=6 \text{ cm}$ , outer radius $R=20 \text{ cm}$ & four identical rectangular slots each having width $5 \text{ cm}$ & length (depth) $15 \text{ cm}$ measured from the center $O$ . Find out the solid angle (in Ste-radian) subtended by slotted section at the point $P$ lying at a distance $OP = 5 \text{cm}$ on the normal axis through the center $O$ (i.e. point $P$ lies normally outwards to the plane of paper.