The playground of our school is in the shape of a quadrilateral ABCD. A flagpost standing vertically in the ground subtends angles of `45^@ and 30^@` at two points P and Q, which lie on the sides AB and AD respectively such that AP = 50 m and AQ = 30 m. If P and Q are the nearest points on the sides AB and AD to the flagpost, then the height of the flagpost is :