Aptitude - Pipes and Cistern - Discussion

Here pipes A and B are used for filling the tanks,
A and B takes 5 and 6 hours respectively to fill the tank .
for 1 hour ,A can fill 1/5th of the tank and B can fill 1/6th of the tank.
now tank C is used foe emptying the tank(water is running off from the tank)..it can empty the tank in 12 hours..means for 1 hour it empties 1/12th of water from the tank

NOTE:
A and B are filling the tank whereas C is used for emptying the tank.
therefore add A and B and subtract it from c
now consider how much water gets filled in the tank in 1 hour,
1/5+1/6-1/12=17/60
for 1 hour----17/60
? hours----1
(note: for 1 hour-- 17 parts got filled up out of 60 parts
for "?"hours-- 60 parts get filled up out of 60 (60/60=1)

Here 1 indicates tank gets filled)
just by cross multiplying it
(1*1)=(?*17/60)
?=(17/60)/(1*1)
60/17 hours it can fill the tank.

Can't understood. Explain clearly.

Why we take 17/60 as 60/17 in the next step ?

@komal
net part in 1 hour is 17/60 part
so in 60/17 hour the part will be 1
here 1 means tank is full.....
its just like the time and wok concept...

Why we take 17/60 as 60/17 in the next step ?

1/12 is smaller dan other two

Why we didnt take ( 1/12 - (1/5+1/6) ). Here 12 is greater right?