Question 446383
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Answer:  <font color=red>15 minutes</font>



Reasoning
1 hr + 30 min = 60 min + 30 min = 90 min


When working alone we have these time durations in minutes only
Jill = 30 min
Jack = 45 min
John = 90 min
The LCM of that set is 90. 


Consider a pool with a capacity of 9000 gallons. This hypoethical value can be changed to anything else and the final answer will be the same at the end.
I'm tacking a few zeros onto 90 to get some large capacity, so when we divide it later on we get integer results.


When working alone, Jill fills 9000 gallons in 30 min. Her rate is 9000/30 = 300 gallons per min.
rate = amountDone/time


When working alone, Jack does the same job in 45 min. His rate is 9000/45 = 200 gallons per min.


And when working alone, John's rate is 100 gallons per minute because 9000/90 = 100


The unit rates for each person are then added up. The assumption is that each person doesn't hinder the other when working together.
Eg: John doesn't slow down Jack
Jill + Jack + John = 300+200+100 = 600
They combine to a rate of 600 gallons per minute.


When working together, the pool gets filled in <font color=red>15 minutes</font> because (9000 gallons)/(600 gallons per min) = <font color=red>15</font>
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