we assume that "1000th" should be interpreted as "1000ml", as in a quantity of 1000 ml of solution; we further assume that the draining is performed first and then the pure acid is added, this ensures that all drained acid is from an 18% solution

Let 'x' represent the amount that should be drained and 'x' therefore also represents the amount of pure acid to be added

pure-acid/quantity = percent solution, so we can calculate the amount of pure acid in the drained solution by this expression (multiplying quantity times percent solution):

(1000-x)*0.18

since we are adding 'x' ml of pure acid to obtain 1000*0.21 of pure acid in the final solution, we have:

(1000-x)*0.18 + x = 1000*0.21

solving for 'x' we have:

x = 36.585

we should therefore replace approximately 36.6 ml of 18% solution with pure acid to obtain 1000 ml of 21% solution