I need help solving this word problem

Question

You need a 15% alcohol solution. On hand, you have a 910 mL of a 10% alcohol mixture. You also have 80% alcohol mixture. How much of the 80% mixture will you need to add to obtain the desired solution?

Answer 1

Let x = the unknown amount of the 80% solution

Then,

{ 910(10) + x(80) }/(910 + x) = 15 This is a weighted average keeping consistent units

and 9100 + 80x = 13650 + 15x

or 65x = 4550

and x = 70 ml of the 80% solution needed