The dimensions of the wood change from 50 by 40 (2000sq cm) change to 0.9*50=45 by 0.5*40=20 area = 900sq cm. 900/2000=9/20, so the area is reduced by 11/20=55%. The question doesn't say in what way the sphere (solid 3-dimensional object) and the wood are to be related. The wood has no given thickness and at best could only cover the surface of the sphere.