Answer:
Mix 50 mL of the 68% solution with 40 mL of the 77% solution to make 90 mL of the 72% solution.
Explanation:
Let the volume required of the 68% solution be [tex]x[/tex] mL. The volume of the 68% solution and the 77% solution shall add up to 90 mL. Thus the volume required of the 77% solution shall be [tex](90- x)[/tex] mL.
The amount of solute in the 72% solution will be:
[tex]90\times 72\% = 64.8[/tex].
The [tex]x[/tex] mL of the 68% solution will contribute:
[tex]68\% \cdot x = 0.68\;x[/tex].
The [tex](90- x)[/tex] mL of the 77% solution will contribute:
[tex]77\% \cdot x = 0.77\;(90 - x) = 69.3 - 0.77\;x[/tex].
The two values shall add up to [tex]64.8[/tex]. That is:
[tex]0.68\;x + 69.3 - 0.77\;x = 64.8[/tex].
[tex]-0.09\;x = -4.5[/tex].
[tex]\displaystyle x = \frac{4.5}{0.09} = 50[/tex].
In other words, there need to be
