suppose you have one quart of water/juice mix that is 40% juice, and you add 3 quarts of water. What percent juice is the final mix?

The percent is 35%.

suppose you have one quart of water/juice mix that is 40% juice,

and you add 3 quarts of water. What percent juice is the final mix?  

You start out with 1.0 quart of mix that's 40% juice, so 

you have 0.4 qt of juice and 0.6 qt of water. 

You add 3.0 qts of water and the mix becomes 

0.4 qt of juice and 3.6 qt of water. 

That resultant mix is a total of 4.0 qt of which 0.4 qt is juice. 

The final proportion of juice is 0.4 / 4.0 = 0.1 which stated as a percentage is 10%  

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