Aptitude - Pipes and Cistern - Discussion

Well explained, thanks @Ayush.

Here if you do it by simple LCM method u will get 32 answers but if you take 1 minute alternate for both u will get 30 answers as;

(3+5) = 2 minute cycle.
=120(lcm of 60 nd 40)/8 = 30minutes.

Thanks @Sanjay Pahade.

Thanks @Viz.

When you consider half the volume the answer is 32. But when you consider half the time the answer is 30.

@Sri, if we try by the method of LCM the answer is 32 only.

Explained as,

A takes 60 mins
B takes 40 mins
taking LCM of both 60 and we get 120.

Considering 120 to be the capacity of the tank.

Now A fills the 120/60 = 2 units/min
B fills 120/40 = 3 units/min.
Combined A and B fill 3+2 = 5 units per min.

Now half of the units is filled by B alone i.e 120/2 = 60 units (half of capacity calculated by taking the LCM).

Therefore, the time taken by B alone is 60/3.

(calculations
3 units = 1 min
1 unit = 1/3 mins
60 units = 60/3 = 20 mins)

So B alone takes 20 mins.

Now for other half,

A and B together fills remaining 60 units.
A and B together fills 3+2 units/min = 5 units/min

(calculation
A+B fills 5 units in 1 min
they fill 1 unit in 1/5 min
further, they fill 60 units in 60/5 = 12 mins)

Therefore they take time = B alone time + time taken by A and B Combined = 20 + 12 mins = 32 mins.

Anyone can give any explanation of @Sri's comment?

We have to consider half time for both b and (a and b).

So if we consider time =x min then..for half time we have to take x/2 so we get x\2(1/40)wich is for b + x/2(1/24)wich is for both a and b.

So taking common we get x/2(1/40+1/24)=1 which 1 means to full whole tank.

Here,

B's 1 minute capacity is 1/40
A and B combined capacity is 1/24 to fill whole tank

Let x is total time
now x/2 X 1/40 { time for which B alone is works
and x/2 X 1/24 { time for which A and B both works

Now add both (as total time to flll the tank) and take common x/2

We get x/2(1/40 +1/24)=1 {1 is for full tank}

this question is confused me because the real story would not be happen like that. please some one to explain it?