Achemist has one solution that is 25% salt and 75% water and another solution that is only 5% salt. how many milliliters of each should the chemist use to make 1400 ml of a solution that is 10% salt?

The formula that we will be using is:

total salt / total solution = .10
Let a be the ml of the first solution

And let b be the ml of the second solution
.25a + .05b / a + b = .10
Also:

a + b = 1400

a = 1400 - b
So substitute:
.25 x (1400 - b) + .05b / 1400 = .10

350 - .25b + 0.05b/ 1400 = .10

350 - .2b / 1400 = .10
350 - .2b = .10 x 1400

.2b = 350 - 140

.2b = 210

b = 1050

a = 1400 - b

a = 1400 - 1050

= 350 mL