288. Grinding Wheel. Three people bought a grinding wheel with a diameter of $20 \mathrm{~cm}$. How much should each of the companions grind off so that the wheel is divided equally, if 4 cm of the diameter is excluded for the hole? The practical value of each share is not considered, only the equal division of the total mass of the wheel.
288. The first companion must use the grinding wheel until its radius is reduced by 1.754 cm. The second must reduce the radius by another 2.246 cm, leaving 4 cm and a hole for the third. This is a very good approximation.