Question 487925
As usual answer = x = mL of 5% acid to be added to 60 mL of 20%.
Focus on acid content:
Old + Added = New
60 * .2 + x * 0.05 = (60 + x) * 0.1
12 + 0.05 x = 6 + 0.1 x
x (0.1 - 0.05) = 12 - 6
0.05 x = 6
x = 120 mL
Which is what one would hope for: that on a see-saw supported at 10, you need 2 units at 5 to balance 1 at 20.