We have 10 ml of 20% acid to which we need to add "x" milliliters of 100% acid to make 50% acid.
.20 * 10 + 1.00x = .50 * (10 +x)
in which "x" is the volume of the 100% acid to be added.
2 + x = 5 + .5 x
.5x = 3
x = 6 milliliters
Source:
http://www.1728.org/mixture.htm
Scroll down to problem B
Z-4.5 = -1.5
Add 4.5 to each side:
z = -1.5 + 4.5z = 3
Cedric is correct because he used the inverse of subtraction and added 4.5.
