We are asked to find out how much of the light striking the gray rectangle is absorbed?
As you can see, 35% of the light got reflected after striking the gray rectangle.
The remaining light is
100% - 35% = 65%
65% of the light went into the gray rectangle and only 15% transmitted through it.
The light absorbed is
65% - 15% = 50%
50% of the light striking the gray rectangle is absorbed.