Question 193595
 A chemist needs to have a 70% acid solution. He has 5 gallons of a 90% acid solution. How much pure water must be added to the 90% acid solution to obtain the 70% acid solution he requires?
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5 gal of 90% --> 4.5 gals of acid (the rest water)
For the 4.5 gal of acid to be 70% of the solution, the total amount of liquid is 4.5/0.7 = 45/7
5 gals is 35/7 so 10/7 gallons of water has to be added.