Diffraction is the spreading out of waves as they go through an opening or around a corner. In this case, the spot on the wall is bigger than the light the came out of the laser because it spread out as it traveled across the room.
Let's say the number is 9. 10% of 9 is 0.9. Add that to 9 and you would get 9.9. 10% of 9.9 is 0.99. Subtract and you would get 8.91. Then you could do the rest.