21-26 must divide evenly by 6 and must represent between 2/3 and 4/5 of 4-21, and therefore must be 12/0, making 26-27 10/0 and 26-28 2/0. 21-22 is either 4/0 or 6/0, and therefore 22-24 is either 1/0 or 2/0.
now let's jump to the cold side for a bit. 9-8 can be 0/30 or 0/36. if it is 0/30, 8-7 can be 0/6 or 0/10. 0/6 would not cool the water in 7 below 80 degrees. if it were 0/10 then the contents of 12 would be 0/20, which is impossible because the outgoing pipes indicate that it must be divisible by 7, 8, or 9. therefore, 9-8 is 0/36 