You need 75 mL of a 85% alcohol solution. On hand, you have a 75% alcohol mixture...

Question

You need 75 mL of a 85% alcohol solution. On hand, you have a 75% alcohol mixture. How much of the 75% alcohol mixture and pure alcohol will you need to obtain the desired solution?

You will need

 _____mL of the 75% solution

and

 _____mL of pure alcohol.

Answer 1

x + .75(75-x) = .80(75)

x - .75x = .80(75) -.75(75)

.25x = .05(75) = 3.75

x = 4(3.75) = 15 ml of pure alcohol mixed with

75-15 = 60 ml of 75%, to get 75 ml of 80%

15+ .75(60) = .15+45 = 60 = .8(75)

75 is within 5 of 80

100 is within 20 of 80

20/5 = 60/15 = 4