How many hours would it take Pump A and Pump B working together, each

E

A)A's rate is there
B)B's rate is given in litre/min

1+2
not suff
We don't know the total time for B to empty tank

Explanation in video format by GMAT Quantum

https://gmatquantum.com/official-guides ... nt-review/

[quote="standyonda"]Hello guys...

I know that this is a simple problem but I have questions.

In many work/rate problems we create the WRT table and after having the 4hrs and 20 min for (1) we assume that the total work equals 1 and we have the rate which is 1/(time of pump A to complete the work). This basically helps us determine the % of completion per hour!

In (2) we are given the actual rate which is understandable that doesn't help since we don't know the total liters or total time.

Q1: Is it wrong to say that the rate for pump A from (1) is 1/(4hrs 20mins) meaning 23% of the work per hour?
Q2: Logically since we know from (1) the rate and from (2) the rate we can find the total time by adding the two rates and find the reciprocal of the result. Why we can't do that?
Q3: If we had on (2) the time of completion the (1+2) would be SUFFICIENT? -- We could use the same logic as (1).

Please let me know. I must have missed something...

I have the exact same question , can anybody shed some light on this issue?
Why can't we approach it by setting 1/260 + 72 =1/x ?
gmatbusters chetan2u Bunuel VeritasKarishma

Please find attached the response to your queries:
Q1: Is it wrong to say that the rate for pump A from (1) is 1/(4hrs 20mins) meaning 23% of the work per hour?
It is valid.

Q2: Logically since we know from (1) the rate and from (2) the rate we can find the total time by adding the two rates and find the reciprocal of the result. Why we can't do that?
from (1), we know the rate of A in terms of total work, from 2, we know the B's absolute rate of doing work but since, we don't know the total work, we can't find the rate of B in terms of total work and hence can't use them as you mentioned.

Q3: If we had on (2) the time of completion the (1+2) would be SUFFICIENT? -- We could use the same logic as (1).
if time of completion of B given,then we can find the rate of B in terms of total work, so using both statements 1 &2, we can find answer, hence sufficient.

gmatbusters thank you for the prompt reply, just one question could you elaborate on the difference between absolute rate of doing work and the rate in terms of total work?

Nice C-trap problem

How many hours would it take Pump A and Pump B working together, each at its own constant rate, to empty a tank that was initially full?

(1) Working alone at its constant rate,Pump A would empty the full tank in 4 hours 20 minutes.
Insufficient b/c we don't know B.
(2) Working alone, Pump B would empty the full tank at its constant rate of 72 liters per minute.
Insufficient b/c we don't know A.

C: Careful, B doesn't actually tell us how long it would take to empty the tank, only that it does so at 72 litres / minute. We would need the actual duration as in A.

E.

Solution:

We need to determine the number of hours it takes Pump A and Pump B working together, each at its own constant rate, to empty a tank that was initially full.

Statement One Alone:

We are given the time it takes Pump A alone to empty the full tank. However, since we don’t know the time it takes Pump B alone to empty the full tank, we can’t determine the time it takes both pumps working together to empty the full tank. Statement one alone is not sufficient.

Statement Two Alone:

We are given the rate of Pump B. However, since we know neither the rate of Pump A nor the full capacity of the tank, we can’t determine the time it takes both pumps working together to empty the full tank. Statement two alone is not sufficient.

Statements One and Two Together:

We are given the rate of Pump B to be 72 liters per minute and we can determine the rate of Pump A to be 1/(4 x 60 + 20) = 1/260 of the tank per minute. However, since these two rates are not in the same units and we don’t know the full capacity of the tank, we can’t determine the time it takes both pumps working together to empty the full tank. Both statements together are not sufficient.

Answer: E

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