<PRE><B>Question 438100</B>
He needs to mix 20 liters of 40% acid solution with a 70% acid solution to get a
 mixture that is 50% acid.
 How many liters of the 70% acid solution should he use? 
:
Let x = amt of 70% solution required
:
Write an amt of acid equation in decimal form:
.40(20) + .70x = .50(x+20)
:
8 + .70x = .50x + 10
:
.70x - .50x = 10 - 8
:
.20x = 2
x = {{{2/.2}}}
x = 10 liters of 70% solution
:
:
Confirm the solution
.4(20) + .7(10) = .5(10+20) 
8 + 7 = .5(30)</PRE>