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I have a question.
Since, we are refilling 3 times, the target customers should be 750 because for first(500 litres) 250 customers, they got real milk without mixture.

Please tell me, what am I missing here.
Thanks.
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The most important part of the question is to understand that we need to find out what is the number of people that the milkman has CHEATED, not the total number of people he sold to.

He first has 1000L of pure milk, of which he sells 500L to customers.
2L per customer means he sells to 250 customers. We don't need to count these customers since they were not cheated because they got pure milk.

He is now left with 500L and adds another 500L of water, making the ratio of Milk to water 1:1
He sells 2L per customer ie 250 customers that have been cheated.

He stops at 500L and adds another 500L of water. The earlier mixture had a ratio 1:1, hence the 500L has 250L of milk and 250L of water. Adding another 500L water makes it 250L of milk and 750L of water which is a ratio 1:3.
He again sells to 2L per customer and cheats 250 customers.

He is left with 500L of adulterated milk and adds another 500L of water to it. The earlier ratio was 1:3, hence now we have 125L of milk and 875L of water. This is exactly 1:7 and hence he needs to sell of all the milk because anymore adulteration will breach the limit of the ratio. 2L of milk per customer for 1000L will yield 500 customers.

Total number of customers cheated

\(250+250+500=1000\)
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Yogananda
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I have a question.
Since, we are refilling 3 times, the target customers should be 750 because for first(500 litres) 250 customers, they got real milk without mixture.

Please tell me, what am I missing here.
Thanks.

No that will not be the case. In this approach, we are not considering the first 500L

When water is mixed for 3 times, then 1500 L of water is added. On top of that, there is 500 L left which will also be sold since the solution cant be diluted any further. Hence it will a total of 2000 L of diluted milk which when divided by 2 gives us the correct answer.

This approach is brilliant but tough to perceive.
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Yogananda
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I have a question.
Since, we are refilling 3 times, the target customers should be 750 because for first(500 litres) 250 customers, they got real milk without mixture.

Please tell me, what am I missing here.
Thanks.


Great question Yogananda , I overlooked that part and the amount of milk sold in total would be 500 + 500 from the first 2 refills, then on the 3rd refill we still have 1000 L of milk to sell resulting in 2000 L of diluted milk sold in total. So I just happened to get the right answer but the logic and the method still applies!
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He started with 1000L

First time
500 removed did not cheat in this case

Remaining mixture

M W

500 500


Second time

500 cheated, ratio of milk to water 1:1


Remaining mixture

M W

250 750



Third time

500 cheated ratio 1;3



Remaining mixture

M W

125 875

Fourth time

500 cheated, ratio 1;7


Remaining mixture

M W

62.5 437.5


Since he cant sell if ratio falls below 1;7 remaining volume 62.5 + 437.5 = 500.


Cheated volume 500 x 3 = 1500


Total = 1500 + 500 = 2000

Therefore number of people cheated = 2000/2 = 1000

Answer E.
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