Pump A can empty a pool in 8 minutes. Pump B can empty a pool in 6 min

Question

Pump A can empty a pool in 8 minutes. Pump B can empty a pool in 6 minutes. Pump A pumps the pool for 2 minutes before pump B joins in. How may minutes will it take to empty the pool together?

A. 5
B. 32/7
C. 18/7
D. 17/7
E. 2

Chethan92 · Answer

Total work to be done = LCM(6,8) = 24 Units.

Pump A can do 24/8 = 3 units in a minute

Pump B can do 24/6 = 4 units in a minute

Pump A works for 2 minutes then 6 units is done. Remaining is 18 units.

Together they empty 7 units in a minute.
Time taken is 18/7 minutes

C is the answer

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Chethan92 · Answer

Bunuel , Shouldn't the answer be 18/7 and not 32/7? 
Please clarify

Archit3110 · Answer

rate of pump A: x/8 and rate of pump B : x/6

combined rate of A+B: 7x/24 and work would be 24x/7

now pump A has worked for 2mins so work done by A : x/8 * 2= x/4 , x=1

work left , 1-1/4 = 3/4

w=r*t
3/4=7/24 *t
time= 18/7
since 2 mins extra are taken by A so 18/7+2 = 18+14/7 = 32/7 IMO B

ShankSouljaBoi · Answer

You forgot to add the two minutes which A has already taken for completing the work.

GMATinsight · Answer

The highlighted part is a mistake

combined rate of A+B: 7x/24 means that work done by A and B together in 1 min = 7x/24 where total work = x

Hence work left after 2 min work of A = x = (x/4) = 3x/4

Now, MT/W = constant
1 / (7x/24) = time / (3x/4)

time = 18/7

Answer: Option C

My way of doing this question 
If B works for t minutes
then A works for (t+2) minutes

Work done by A in (t+2) min = (t+2)/8
Work done by B in (t) min = (t)/6

Total Work Done = (t+2)/8 + (t)/6 = 1

i.e. t = 18/7

Archit3110

ikeleonard · Answer

Let x be the time it takes A and B to empty the pool
Rate of Pump A = 1/8
Rate of Pump B = 1/6
Pump A pumps the pool for 2 minutes => 1/8* 2 = 1/4
Pump B joins in to empty the pool => 1/4 +   = 1
(7/24)x = 1 - 1/4 = 3/4
x = 18/7
Total time to empty the pool => 18/7 + 2 = 32/7
Answer = B

Archit3110 · Answer

GMATinsight : thank you sir... I have understood where I went wrong..

,

Kem12 · Answer

I feel the question is poorly worded... it's says how many minutes to empty the pool "together". However, I would like to ask: Can the GMAT use such subtle wording that allows ambiguity, in wording an official question? Bunuel...

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pandeyashwin · Answer

Gmat questions are not ambiguous. This question is either poorly worded or the provided OA is incorrect.

AnirudhaS · Answer

How may minutes will it take to empty the pool  together ?

together might also mean just =  \frac{1}{1/6+1/8} = \frac{24}{7}

It makes sense to me that answer is C, not B. Because the first 2 minutes of work are not done  together.

Hea234ven · Answer

This question is asking for the time taken by both A and b together, not together+ A separately. So and should be c, not b.

randomavoidplease · Answer

Umm seems wrong but I am still learning so let me know.

Machine A  \frac{1}{8} * 2 + t = \frac{2+t}{8} 
Machine B  \frac{1}{6} * t = \frac{t}{6}

So finally:

\frac{2+t}{8 }+ \frac{t}{6} = 1

To solve and we get  \frac{18}{7}  Minutes

Ans :  C

How is B the right answer?

A worked for 2 minutes   then   B joined . Thus A worked for 2 + t minutes and b worked for t minutes.

fireagablast · Answer

TBH i don't think the question is worded explicit enough.
The question states "How may minutes will it take to empty the pool together?". That can either mean "how long do A and B work together" or "what is the total time taken to empty the pool".

If you're asking for the total time - you should just say "How long will it take to empty the pool" or "What is the total time taken to empty the pool" and not use language that can be interpreted multiple ways.

Pump A can empty a pool in 8 minutes. Pump B can empty a pool in 6 min

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Pump A can empty a pool in 8 minutes. Pump B can empty a pool in 6 min

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