Question 1011852
A 40% solution is to be mixed with water how much water is needed to make the solution 10%? 
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If it's a 40% solution, it is 60% water.
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Assume you start with 100 gallons of the 40% solution.
60 gallons of it must be water
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You are going to add "x" gallons of the water to the solution.
You now have (100+x) gallons of solution.
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You want that to be a 10% solution.
You have to solve for "x".
Equation:
water + water = water
60 gal + x gal = 0.90(100+x)
60 + x = 90 + 0.9x
0.1x = 30
x = 300 (number of gallons of water you would have to add)
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You started with 100 gallons of solution; 60 gallons was water
You added 300 gallons of water so you have 400 gallons of solution
40 gallons of active ingredient are still in the solution.
And 40 is 10% of 400.
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The original solution was 4 out of 10 or 2/5 active.
The new solution is 4 out of 40 or 1/10 active
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The mixture is now 10 inactive  to 1 active.
Cheers,
Stan H.