Edward C. answered • 02/13/15

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Let T = liters of 25% acid

Let F = liters of 50% acid

We want 30L total ==> T + F = 30 ==> F = 30 - T

We want the final solution to be 30L that is 40% acid ==>

it contains 30L * 0.40 = 12L acid

The total amount of acid in the mixture of T liters of 25% acid and F liters of 50% acid is given by 0.25T + 0.50F

So our 2nd equation is 0.25T + 0.50F = 12

Substitute the value for F from the 1st equation in to this equation and solve for T

0.25T + 0.50(30 - T) = 12

0.25T + 15 - 0.50T = 12

-.25T = -3

T = 12

So F = 30 - T = 18

We should mix 12 liters of 25% acid with 18 liters of 50% acid

Check: 12L + 18L = 30L total

0.25(12) + 0.50(18) = 3 + 9 = 12L acid in mixture so mixture is 12/30 = 40% acid