George wishes to increase the percent of acid in 50ml of a 15% acid solution in water to 25% acid, how much pure acid must you add?

Amount times concentration=quantity

50(0.15)+x(1)=(50+x)(0.25)
x=amount of acid to add

so
7.5+x=12.5+0.25x
minus 0.25x both sides
7.5+0.75x=12.5
minus 7.5 both sides
0.75x=5
divide both sides by 0.75
x=6.666666666
converts to 6 and 2/3

you have to add 6 and 2/3ml of pure acid

paul's with a y-intercept of $ 2856