Question 313071
How many mL of 20% acid solution must a scientist add to 80 mL of 5% solution to create a 10% acid solution?

amount A * %A + amount B * %B = amount C * %C

x * 0.20 + 80 * 0.05 = (x + 80) * 0.10
0.20x + 4 = 0.10x + 8
0.10x     = 4
    x     = 40 ml