Aptitude - Pipes and Cistern - Discussion

i did not understand method plz help

Its very simple.

Part filled in 1st and 2nd hour:

1/12+1/15+1/12+1/20 = 17/60.

Part filled : Time taken
17/60 : 2 hr
1 : ?

By solving this we will get 6.8 that is nearby 7.

Please carefully read the question A is open for all times, B and C are opened alternatively for one hour, that means if B is opened for 1st hour that time C is closed(A and B filling the tank) For 2nd hour B is closed C is opened(A and C is Filling the tank)3rd hour B opened C closed and so on...

1st hour:

1 hour work of A and B is 1/12+1/15 = 3/20.

2nd hour:

1 hour work of A and C is 1/12+1/20 = 2/15.

3rd hour:

1 hour work of A and B is 1/12+1/15 = 3/20.

4th hour:

1 hour work of A and C is 1/12+1/20 = 2/15.

5th hour:

1 hour work of A and B is 1/12+1/15 = 3/20.

6th hour:

1 hour work of A and C is 1/12+1/20 = 2/15.

7th hour:

1 hour work of A and B is 1/12+1/15 = 3/20.

(Until you have to calculate 60/60).

If we add the whole we will get 1 is the answer that is capacity of tank.

3/20+2/15+3/20+2/15+3/20+2/15+3/20+2/15+3/20 = (9+8+9+8+9+8+9)/60 = 1.

Hence, time taken to fill the tank is 7 hours.

(A + B)'s 1 hour's work= 1/12+1/15 = 3/20.

(A + C)'s 1 hour's work =1/12+1/20 = 2/15.

Therefore (3/20 + 2/15)= 17/60 part of work is completed in first 2 hrs.

17/60 part of work completed in ------- 2hrs.

1 part (full work) completed in ------- x hrs.

Now cross multiply => x= 2(60/17) = 7hrs.

It written part filled in six hours then why is it multiplied by 3 and not six?

Take LCM a, b, c = 180, tank capacity 180 liters.

a - 15 liter per hrs.
b - 12.
c - 9.

1hrs a+b = 27 liter.
1hrs a+c = 24 liter.

In choice more than 6 hrs.

= 27*4 = 108.
= 24*3 = 72.

Total 180 liters in 7 hrs.

I feel its all lengthy calculation. Is there any simple shortcut method?

Yes, take a bucket of 60 ltr capacity. Install 3 taps in your bathroom and get started.

1 hr tank can fill 3/20 -> (1/12+1/15).

2nd hr tank can fill 3/20+2/15 = 17/60.

For 2 hr it fills 17/60 then for 1 hr it fills 17/120 -> (1/2*17/60).

Consider 17 table 17*6 = 102, so in six hours it it fills 102/120 and next hr B Pipe will be open and.

Fills as 102/120+3/20 = 1. So 7 hrs it will take.

Another way:

For 2 hrs it fills 17/60.

Consider next 2 hrs i.e. 4 hrs 2*17/60 = 34/60.

Consider next 2 hrs i.e 6 hrs 51/60 only 9 is less for 60 i.e to full.

So next 1 hr b pipe open 51/60+9/60 = 1.

Please say short cut method for problem 14 it is length process.