A math question I have no clue how to solve

Question

You need a 70% alcohol solution. On hand, you have a 260 mL of a 20% alcohol mixture. You also have 90% alcohol mixture. How much of the 90% mixture will you need to add to obtain the desired solution?

Answer 1

Let x = amount of 90% solution

0.20(260) + 0.90x = 0.70(x + 260)

52 + 0.90x = 0.70x + 182

0.20x = 130

x = 650 milliliters