Mixture problem

Question

You need a 40% alcohol solution. On hand, you have a 105 mL of a 25% alcohol mixture. You also have 75% alcohol mixture. How much of the 75% mixture will you need to add to obtain the desired solution?

Answer 1

We use percent time volume to get moles of the substance. let X be the amount of 75%solution.

we need 0.25(105) + 0.75x = 0.40(105 + x). The rest is just algebra.