When two people or things "work together" it is their rates that add. "A tank can be filled by one of two pipes in 12.00 h " means that the filling rate for that pipe is 1 tank/12 h= 1/12 "tanks per hour". "by the other of the two in 1.00 h." means that its filling rate is 1 tank/1 hour= 1 "tanks per hour". Be careful about that third pipe- it is draining, not filling, the tank. That simply means we can treat it "filling" rate as negative. Since it empties the tank in 10.4 hours, its "filling" rate is -1/10.4 "tanks per hour". With all three pipes open their filling rate is 1/12+ 1- 1/10.4 "tanks per hour". You can either convert to decimals by dividing or treat that "1/10.4" as "10/104". Once you have the filling rate of all three in "tanks per hour", the time to fill one tank, "hours per tank" is just the reciprocal.

Same idea. Bill does the job in 3.8 h so his working rate is 1/3.8 "jobs per hour". John does the job in 5.4 h so his working rate is 1/5.4 "jobs per hour". Working together, their rate is 1/3.8+ 1/5.4. Once you have added those and have their working rate together in "jobs per hour", the time to do one job, "hours per job" is the reciprocal.

No, you do NOT average the times!
(Added: well, not the arithetic average. You ARE doing what is called the "harmonic average". The harmonic average of the two numbers a and b is . You find the arithmetic average of 1/a and 1/b and and invert that.)