In the figure , a square OABC is inscribed in a quadrant OPBQ . If OA =20 cm , find the area of the shaded region.  <br> <img src="https://doubtnut-static.s.llnwi.net/static/physics_images/AGP_EDG_MAT_X_BSC_SQP_09_E01_056_Q01.png" width="80%"> 