How many ml of 30 percent acid should be added to pure acid to make 70 ml of 35 percent acid?

  65 mL

Step-by-step explanation:

Let x represent the quantity (mL) of 30% acid that should be added to (70-x) mL of pure acid to make 35% acid. The quantity of acid in the mix is ...

  0.30·x + 1.00·(70 -x) = 0.35·70

  -0.70x = 70·(-0.65) . . . . . . simplify, subtract 1.0·70

  x = 65 . . . . . . . . . . . . . . . . . divide by the coefficient of x

65 mL of 30% acid should be added to 5 mL of pure acid to make 70 mL of 35% acid.

Comment on the problem wording

Safety precautions should be observed when working with pure acid. As a chemistry problem (not a math problem), 5 mL of pure acid should be added to 65 mL of 30% acid, not the other way around.