A circular coaster of the largest possible size is cut from a square wooden tile. What percentage of the tile is wasted? (Use π = 22/7 and give the answer correct to two decimal places.)

Let the tile's side be $2r$, so its area is $4r^2$ and the largest circle inside has radius $r$ and area π$r^2$. The waste fraction = 1 - π$r^2/4r^2=1$ - π/4 = 1 - 22/28 = 6/28 = 0.2143, i.e. 21.43%. The result does not depend on the tile's size at all. The trap 78.57% reports the share KEPT as the coaster rather than the share wasted; 25% guesses that the four corner pieces make a quarter of the tile; 50% assumes the circle fills half of it.