Need help finding a equation and step to solving the solution.

You need a 15% alcohol solution. On hand, you have a 105 mL of a 35% alcohol mixture. How much pure water will you need to add to obtain the desired solution?

A) Write an equation using the information as it is given above that can be used to solve this problem. Usex as your variable to represent the amount of pure water you need to use.

B) You will need ____________ mL of pure water to obtain ______________ mL of the desired 15% solution.