You need a 50% solution. On hand, you have a 200 mL of a 35% alcohol mixture. You also have 70% alcohol mixture. How much of the 70% mixture will you need to add to obtain the desired solution?

150 mL

Step-by-step explanation:

We need a mixture that is at 50%, now we have 200 mL at 35% and another mixture that is at 70%, that is, we must calculate how much of this mixture is added, by the following equation

200 * 35% + x * 70% = 50% (x + 200)

70 + 0.7 * x = 0.5 * x + 100

0.7 * x - 0.5 * x = 100 - 70

0.2 * x = 30

x = 30 / 0.2

x = 150

150 mL must be added so that the mixture is 50%.