Question

how many liters of a 10% solution must be mixed with 5 liters of a 50% solution to give a 20% solution?

Answer 1

The amount of solution can be found by multiplying number of liters by percentage.

$x*10\% + 5*50\% = (x+5)*20\% \\ \\ 0.1x +2.5 = 0.2(x+5)=0.2x + 1 \\ \\ 0.2x -0.1x = 2.5 -1 \\ \\ 0.1x = 1.5 \\ \\ x = 15$

So 15 liters of 10% solution mixed with 5 liters of 50% solution yield 20 liters of 20% solution.