How many milliliter of pure acid should be added to 10 ml of 20% acid to make 50% acid solution? round answer to the nearest hundredth if necessary

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

If you mean width, 7 x 7 = 49, so 49 feet, then add on the 3.5 feet due to the 0.5 left over, and you have 52.5