Each mirror absorbs about 30% of the EUV incident upon it (and the mask counts as a mirror). With about 9 total mirrors on the path, you're down to 0.7^9 ~= 4%

Each mirror absorbs about 30% of the EUV incident upon it (and the mask counts as a mirror). With about 9 total mirrors on the path, you're down to 0.7^9 ~= 4% of the initial light left.