How to start the problem i don’t understand how they got 20n

Awork system has five stations that have process times of 5, 9, 4, 9, and 8. what is the throughput time of the system?

Which property is illustrated by the following statement? if hxa rgi, then rgi hxa.

Awater tank holds 18000 gallons. how long will it take for the water level to reach 6000 gallons if the water is used at anaverage rate of 450 gallons per day