Question 980554
What are your materials?  If you add some amount of water to 70% alcohol, then you will obtain a concentration of alcohol LESS THAN 70%.


If you are starting with 100% alcohol and want to add some water to prepare 25 fluid ounces of 70% alcohol, then ...


v, volume of the "pure" alcohol of 100%;
25-v, volume of the water to add;


{{{(100*v+0*(25-v))/25=70}}}


{{{(100v)/25=70}}}
Find v, and then find 25-v.