I need help with this problem.

Question

You need a 40% alcohol solution. On hand, you have a 400 mL of a 30% alcohol mixture. You also have 90% alcohol mixture. How much of the 90% mixture will you need to add to obtain the desired solution?

Answer 1

.9x + .3(400) = .4(400+x)

.9x -.4x = 160-120

.5x = 40

x =40/.5 = 80 ml of 90% mixed with 400 ml of 30% yields 480 ml of 40%

.9(80) +.3(400) = 72+120= 192 =.4(480)