Bunuel wrote:
Soda concentrate is being added into a soda-making machine in a manufacturing plant. The concentrate is added at a constant rate. At the same time prepared soda is being pumped out of the machine at a constant rate. What is the amount of soda being pumped out per minute?
(1) The soda machine has a capacity to hold 1000 gallons of water.
(2) 25 gallons of concentrate are added every minute and the total volume of liquid in the machine is reduced by 75 gallons every minute.
We'll first break our question stem into small bits so it is easier to turn into equations and understand.
This is a Precise approach.
concentrate = added at constant rate, let's call this c
prepared soda = removed at constant rate, let's call this s
so in total volume change is c - s
We need to find s.
(1) this doesn't tell us anything about the rates.
Insufficient.
(2) so c = 25 and total volume change is 75, that is c - s = -75. Substituting c = 25 gives s = 100.
Sufficient!
(B) is our answer.
**note that this answer assumes that the only thing pumped out of the machine is prepared soda and the only thing pumped in is concentrate. This is usually made explicit in the question but, if it isn't, is a reasonable assumption. If you find yourself inventing new data - you're doing something wrong!
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