A 34% alcohol solution is mixed with 15 milliliters of a 40% alcohol solution to make a 37% alcohol mixture. How many milliliters of the 34% alcohol solution were used?

Let $LaTeX: \displaystyle x$ be the amount of the 34% alcohol solution. This gives the equation $LaTeX: \displaystyle 0.34(x)+0.4(15)=0.37(x + 15)$ . Solving for $LaTeX: \displaystyle x$ gives 15 milliliters.