let's recall that there are 16oz in 1 lbs, so then 12lbs is 12*16 = 192oz, plus 5, that makes it 197oz, so then 12lb 5oz is really 197oz.
likewise, 7lb is 7*16 = 112oz, plus 10 that makes it 122oz.
197 - 122 = 75 oz
and 75 oz is just 16+16+16+16+11, 4lbs and 11 oz.