Question

A chemist mixes some 20% acid solution with pure 100% acid to increase the concentration. How much pure acid and acid solution should be mixed to form 100 milliliters of 40% acid solution?

Answer 1

first, represent:
let x = amount of 20% acid used (in mL)
let 100-x = amount of 100% acid used (since the total volume of the 2 acids mixed must be 100)

therefore,
0.2x + 1(100-x) = .4(100)
*note: 0.2 = 20%, 1 = 100%, .4=40%
0.2x + 100-x = 40
-0.8x=-60
x = 75 mL of 20% acid, and
100-x = 25 mL of 100% acid

so there,, i hope i was able to help.. =)