Aptitude - Pipes and Cistern - Discussion

@Rohit are you kidding bro?

If A and B take more than 4 hours to fill the tank (they takes 6 and 8 hours). Then how could anyone of them fill it in 4 hours. Use some mind.

It is easy. It is given that the cistern will be filled in just 30 minutes. And we don't know that who between A and B takes how much time from 30 minutes to fill the tank.

Suppose A takes x minutes. Then B will take (30-x) minutes.

But here is given that B is turned off in some time. We don't know when. So take it as Y minute. After Y minute only A will fill remaining part in 30-Y minute.

So first A+B will fill for Y minute and then B is turned off. So only A will fill in 30-Y minute.

So the equation is (A+B)Y + A(30-Y) = 30 minute.

Pipe A fills tank in 6 hours, pipe B in 8 hours. Both pipes opened simultaneously, then after how many hours should pipe B should be closed so that tank is filled in 4 hours.

I am getting a negative value and answer I am getting is 16/3 which is wrong. Please help.

Let us understand the problem first, we can easily analyze that Here Pipe A is run for HALF an hour.

Total part filled by part A alone in half an hour is {2/75}*30 = (4/5).

Now (1- (4/5) ) , that is (1/5) part is still remaining. And it is filled by Pipe B and after that it is made off.

So (1/5) part filled by Pipe B in,

= 45*(1/5) = 9 minute.

Hence after 9 minute pipe B will made off.

Hey,

After first step just open the brackets i.e,

2x/75 + x/45 + 60/45 - 2x/75 = 1.

(2x/75 - 2x/75) + x/45 + 60/45 = 1.

0 + x/45 + 4/5 = 1.

Take 1/5 as common,

x/9 + 4 = 5.
x/9 = 1.
x = 9.

No need of complex calculations.

Simply.

(1-Given time/A's time)*B's time.

= (1-30/75/2)45.
= (1-60/75)*45.
= (1-0.8)*45.
= 0.2*45 = 9.

Please tell me how we get 2/75?

This is not logical reasoning. By multiplying them with minutes, you can not estimates their unit to be a part.

Hence, the "=1" here is unaccepted. You must provide with minutes unit too.

I still don't get the logic. Why you talk about minutes needed and suddenly talk about part.

You can't write it "=1".

If we reverse the answer to the common formula:

9x11/225+21x2/75.

We will get answer = 1.

But we actually want to get answer = 30 minutes, right?

This explanation is clearly not logical and misleading.

We can't assume minutes unit is same with part unit.

You can't multiply them with minutes and suddenly make the equation "=1" because it's clearly not same unit.