Question 7860
Let x = the number of liters of antifreeze that must be replaced. Change percentages to decimals.

Start with 10 liters of 30% antifreeze.  Write this as: 0.3(10)
You want to remove x liters of 30% antifreeze.  Write this as -0.3(x)
Then you want add x liters of 100% antifreeze.  Write this as +1(x) or just +x.

Now all of this must add up to 10 liters of 50% antifreeze.  Write this as 0.5(10).
Putting it all together, we have:

{{{0.3(10) - 0.3x + x = 0.5(10)}}}  Simplify and solve for x.

{{{3 + 0.7x = 5}}}

{{{0.7x = 2}}}

{{{x = 2.86 }}}

You must remove 2.86 liters of 30% antifreeze then add 2.86 liters of 100% antifreeze.