In my copy the exact line is as follows: The quantization formula ppmtopgm uses is y = .299 r + .587 g + .114 b. so, it's either fixed, or it's a typo to

In my copy the exact line is as follows: The quantization formula ppmtopgm uses is y = .299 r + .587 g + .114 b. so, it's either fixed, or it's a typo to begin with.