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bkoulibali
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A certain quantity of 40% solution is replaced with 25% 02 Aug 2004, 09:29
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A certain quantity of 40% solution is replaced with 25% solution such the new concentration is 35%. What is the fraction of the solution that was replaced?



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A certain quantity of 40% solution is replaced with 25% 02 Aug 2004, 10:00
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B.

You need 2/3 of the 40%
and 1/3 of the 25% to get 35%.

So 1/3 of the original solution was replaced.
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Karthik
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A certain quantity of 40% solution is replaced with 25% 03 Aug 2004, 06:56
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Let X be the fraction

0.4S - X(04.S) + X(0.25S) = 0.35S

you get X= 1/3
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venksune
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A certain quantity of 40% solution is replaced with 25% 03 Aug 2004, 07:38
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Else we can approach it using simple numbers

Let there be 10 litres. 40% solution will be 4 litres. The 25% solutions has 2.5 litres. We are taking out X litres out of it (and replacing it with the same X quantity). The X litres that is taken out has the 40% solution (to the extent of 40%) and the X litres added from the 25% solution has 25% solution (to the extent of 25% of X) in it. The 25% solutions has 2.5 litres. We now have

4 - 4X + 2.5X = 3.5

X = 1/3
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ian7777
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A certain quantity of 40% solution is replaced with 25% 03 Aug 2004, 08:13
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venksune
Else we can approach it using simple numbers

Let there be 10 litres. 40% solution will be 4 litres. The 25% solutions has 2.5 litres. We are taking out X litres out of it (and replacing it with the same X quantity). The X litres that is taken out has the 40% solution (to the extent of 40%) and the X litres added from the 25% solution has 25% solution (to the extent of 25% of X) in it. The 25% solutions has 2.5 litres. We now have

4 - 4X + 2.5X = 3.5

X = 1/3
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Yup. I'm on board with this explanation. I would do this problem by plugging in a number and going with it, though I would choose 100 just because that means there wouldn't be any decimals anywhere.



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