Answer:
You need to add 150 mL of 65% alcohol solution.
Step-by-step explanation:
You have 300 mL of 20% solution.
300 mL of 20% alcohol solution has 20% * 300 mL of alcohol.
You have 65% solution.
Let the volume of 65% solution you add be x.
In 65% solution, 65% of the volume is alcohol, so the amount of alcohol in x amount of 65% solution is 65% * x.
You want 35% solution.
The total amount of 35% solution you will make is 300 mL + x. The amount of alcohol in that amount of solution is 35% * (x + 300).
Equation of alcohol content:
20% * 300 + 65% * x = 35% * (x + 300)
60 + 0.65x = 0.35x + 105
0.3x = 45
x = 150
Answer: You need to add 150 mL of 65% alcohol solution.
