what "answers" did you get, and how did you get them?i hav gotten different ansers for tis poblem
here is the set-up I would use ...
let x = amount of pure HCl in mL
x(100%) + 40(50%) = (x+40)(75%)
solve for x.
let:
x = the amount of pure hydrochloric acid (100%=1)
(40+x) =total amount
x(1)+ 40(0.5) = 40+x(0.75)
x(100%) amount of pure acid + 40ml(50%) solution = 40 + x(75%)
x+ 20 = 30 + 0.75x
x - 0.75x = 30 -20
0.25x = 10
x= 40 ml
let's check it
40(1) + 40(.5) = 40+40(.75)
40+20= 80(.75)
60=60