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You can solve the problem algebraically and/or intuitively.
Intuitive solution is this:
On each gallon of the 10% solution take 3 gallons of the 30% solution.
It works, since
=
= 0.25 = 25%.
Thus you get 5 groups/packages each containing (1 gallon of the 10% solution PLUS 3 gallons of the 30% solutions),
which gives you 5 gallons of the 10% solution AND 15 gallons of the 30% solutions, in all.
Algebraic solution is this:
= 0.25.
saying that the concentration of the mixture is 25% = 0.25.
The steps to solve it are
0.1x +0.3*(20-x) = 0.25*20
0.1x + 6 - 0.3x = 5
-0.2x = 4 - 5 = -1 ====> x =
= 5.
You get the same answer: 5 gallons of the 10% solution and (20-5) = 15 gallons of the 30% solution.
Solved.