Question 844202
The first question, making the 40% alcohol from the given 95% alcohol:
Let v = Amount of water to use.
Start with 135 ml. of 95% alcohol.
{{{(135*95)/(v+135)=40}}}



The second question, making 10% alcohol from the given 40%; a little different because 40% alcohol is being used and diluted with water.  The final concentration wanted is 10%, resulting in 250 ml of this 10%.
Realize that water is 0% alcohol.
Let w = Amount of water to use.
Let v = Amount of 40% to use.
{{{40*v/250=10}}}  AND {{{v+w=250}}}


Find more thorough information on these mixture problems in the lessons.
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http://www.algebra.com/tutors/Two-Part-Mixture-with-one-material-quantity-unknown.lesson?content_action=show_dev
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http://www.algebra.com/tutors/Mixtures%3A-All-in-Symbols.lesson?content_action=show_dev