Question 23368
You can write this out in words then do the algebraic equation.

You want to add x ml of 60% solution to (50-x) ml of 90% solution to obtain 50 ml of 78% solution. So, after converting the percentages to their equivalent decimal values,in algebrese (I just invented this word) you can write:

{{{x(0.6) + (50-x)(0.9) = 50(0.78)}}} Simplify and solve for x.
{{{0.6x + 45 - 0.9x = 39}}} Collect like-terms.
{{{-0.3x + 45 = 39}}} Subtract 45 from both sides of the equation.
{{{-0.3x = - 6}}} Divide both sides by -0.3
{{{x = 6/0.3}}}
{{{x = 20}}}

You would need to mix 20 ml of the 60% solution with (50-20 = 30) ml of the 98% solution to obtain 50 ml of 78% solution.

Check:
20(0.6) + 30(0.9) = 12 + 27 = 39
50(0.78) = 39.