Aptitude - Pipes and Cistern - Discussion

Why can't we solve it in this way:
Let c=3rd pipe, b=2nd pipe, a=1st pipe.

Let us fix c's time = x.
c=x ; a=x+9; b=x+4.

(a+b)'s time =c's time.
1/(x+9) + 1/(x+4) = 1/x.

By solving we get x=6.

Now a=9+6=15.
Ok I think you have understood.

Why neglecting 3 in the answer please can any one say?

Can you explain that 1/x-9 how come to right hand side positive.

Its positive on left side so it may be come to the right side than the equation -1/x-9.

Please any body explain.

Solve through options. It will be easier.

Here why we can not consider x+5? I did not understand this problem can any please explain?

2nd pipe is 5 hours faster than 1st.so 2nd pipe=5x

1st pipe automatically takes 1 hour to fill the tank.1st pipe=1x.

in question 2nd pipe is 4 hours slower than 3rd pipe.

i.e 5x+4x=9x

3rd pipe =9x

add all 3 pipes i.e 1x+5x+9x=15.

This question is a very easy one.

Let 2nd pipe's speed be x.

So 1st pipe's speed will be x+5.

And 3rd pipe's will be x-4 (as per the ques).

Now, it is given in the question that The first two pipes operating simultaneously fill the tank in the same time during which the tank is filled by the third pipe alone.

Hence 1/x + 1/x+5 = 1/x-4.

Solving this we get x = -2 and 10. Obviously x = 5 therefore speed of 1st pipe is 10+5 = 15.

Please somebody tell me in easier way.

Looking at the question, it is quite evident that the first two pipes simultaneously fills the tank in same time.

Assuming the time taken by pipe A to fill the tank is X hours, then for every hour, it will fill 1/X of the tank.

Now, "second pipe fills the tank 5 hours faster than the first pipe and 4 hours slower than the third pipe" --> So the second pipe will require lesser time than pipe A to fill tank i.e. (X-5) hours.

Second tank is also 4 hours slower than the third pipe, so third pipe will be X-5-4 = X-9 hours.

Summing up, the equation goes like:

1/X + 1/X-5 = 1/X-9.

Just remember these:

1. Faster fil = lesser time.

2. If total time=x, then for 1 hr it is 1/x.

3. When you get two different answers after solving the equation just substitute the answers (like x=15) in the original equation and check whether it is feasible.